So from the top
You did your research, you read ACTUAL technical reviews on the gun you were looking at (not video descriptions of what comes with the gun), you purchased it, and it JUST got to your door 5 minutes ago!
So now what??
Well firstly, don't panic when the gun fails to turn over or only gets 10 shots out with the battery that came with it. Batteries are never shipped fully charged for safety reasons.
Lesson #1; Buy a good battery and charger
The battery in your AEG is like the fuel tank in your car, except it also determines what type of fuel is in there. Most cheap chargers and just about all wall chargers, will likely take 8-12 hours to charge your battery and typically not nearly to capacity. Just as well, most guns don't come with good batteries to begin with (A Chinese company wants to save money, imagine that...)
Most of us vets will recommend the iMax B6AC (because it plugs right into the wall) or iMax B6 (requires a power supply but you have the option of hooking it up to a car battery at games). They're very good quality, inexpensive smart chargers that charge NiCd, NiMH and LiPo batteries, for when you inevitably make the switch to LiPo.
Please use google to determine what shape and size of battery your gun requires.
Remember your battery contains your gun fuel, and a bad charger means a less reliable gas tank. And no gas, means no game.
So you've got a good charger, you know the gun will last a day of playing. What about ammo?
Lesson #2; be wary of bad munitions
Just like you wouldn't feed your $200+ dog crap in a bag, you shouldn't feed your $200+ gun crap ammo!
Some guns will run just fine on lower quality ammo, I myself used low quality "flying colors" in a number of AEGs, including some with tightbores with no issues. But be warned right now, lower quality ammo will result (at best) in lower range and accuracy. At worst, it will cause your gun or mags to jam or misfeed.
.12s are notably the worst quality and most useless round. Do not buy these.
As well I should note silica and metal BBs are banned at most fields.
Most common BBs are styrene (white plastic) and Bio (eco-friendly), maybe someone will make a thread about which BBs are good and which are bad some day O_O
One of the most common questions we hear is "How do I make my gun more accurate and get better range?" well step 1 is to use high quality ammo! (Step 2 is to upgrade the hop rubber!)
Maybe you think you know better than I do and you chose to ignore lesson #2;
Lesson #3; what to do if your gun jams
Everyone will have this happen to them at some point in their airsoft career. The single most important thing to do when you have a jam is STOP PULLING THE DAMN TRIGGER.
It may be difficult at first for you to distinguish the sound of your gun firing a BB vs it blank firing, but what's unmistakable is the choking sound it makes when it's jammed, it's very unique and you'll notice it right away.
A very common breakdown for beginners is a self inflicted one; their gun jams, and they think continuing to pull the trigger will somehow dislodge the BB. Continuing to shoot will most likely result in you stripping the piston. Shooting in full auto will guarantee stripping your piston.
Stop shooting. Get a cleaning/unjamming rod, and use that beveled edge to clear the jam, or ask a vet for assistance, we're always glad to help!
This touches back on why you should buy good ammo, the most common cause of jams is poor ammo. You may think saving $5 on a bag of ammo is beneficial, but if it's really bad ammo and it breaks your gun, well I charge $30 to change a piston plus parts. So you're out about $45 and now you need to buy another bag of ammo.
Lesson #4; In case of misfeeding
Another hot topic for beginners; "My gun misfeeds, what should I do?"
In short, see lesson #2.
Otherwise, you may have purchased bad mags that don't work well with your gun, or it may be an operator fault like your hop is set too high, or it's all the way off. Which brings us to;
Lesson #5; Maintenance and lubrication!
So as you may have already noticed, there are a LOT of moving parts in your mechbox, and maybe some on your gun too!
Surely there's some kind of 12pt lubrication chart or schedule that needs to be followed!
Well actually there is, but it's only one point
GENERIC AEG LUBRICATION SCHEDULE
#1 - do not lubricate anything
There is literally nothing outside the mechbox on the entirety of the gun, mags and all, that requires any lubrication (by you)
Being technical, there may be some bits that need greasing, like complex trigger mechanisms or slide plates, or charging handle components, but for the most part you don't need to be lubricating anything on your gun. Yes the mechbox does require occasional lubrication and it's best to replace the factory garbage, but probably best that you (as a beginner) not be poking around in the mechbox right away.
The hop chamber and barrel assembly
Needs to be cleaned/swabbed regularly. Preferably after every game with a tightbore barrel, really depends how much ammo you use. Don't be one of those people that neglect to clean their barrel and come to me with tall tales of compression problems and I end up charging them $20 to spend 2 minutes cleaning their barrel.
To give you a baseline, my high accuracy bolt action rifle would have a noticeable 5% loss in accuracy after just 100 rounds or so. Wider bore barrels will be less affected by fouling. Some people go a year without cleaning but I highly recommend against that.
Grease from your cylinder will also, naturally, spit out into your barrel over time, another good reason to clean it after every game.
Clean the barrel by turning your hopup OFF, then using your cleaning rod with a swab dampened with WINDEX, swab the barrel once. Repeat with a new swab until it comes out clean, then use a dry swab and a twisting action to dry it out.
DO NOT LUBE THE BARREL. The BB is not intended to ride or touch any surface of the barrel. Air pressure keeps it centered in the barrel and ideally prevents it from touches the walls. Lubrication is tantamount to fouling, which will reduce laminar airflow and accuracy.
Silicone will leave a semi-greasy film on the inside of your barrel which will trap dust and dirt from the BB's.
Also, the hop up system works entirely on friction, lubrication reduces friction and neutralizes the hopup.
Should I lube the mags?
In short, no. Never. There are some cases in which people claim this has helped alleviate jams, that's most likely because it helped loosen the dirt that was jamming it up.
Lube gets on the BBs and into the barrel and hop rubber. Loaded up with the crap it's accumulated from the mag, it accelerates barrel fouling.
The spring and follower are actually smaller than the BB channel so they shouldn't be getting caught up on anything. And the BB, being a spheroid, only ever has 2 contact points with the channel and one with the follower, and always in the same spot. So as soon as you empty the mag, there's likely no lube left at those contact points anyway.
You may sometime have to CLEAN a mag, but there's plenty of cases of people having used mags for 3-7 years without ever having to clean or lube them. I myself had 16 king arms 68rnd mags that lasted 5 years with me, never having had any issues, before I sold them, still no issues with the new owner.
The most common problems with mags;
-see lesson #2
-they might not be fully compatible with you gun (IE: KA mags work better than G&P or CA mags in a G&P gun)
-It may just be a poor quality mag, the follower may be getting caught up on dirt or flash (plastic bits left over from casting), most people seemed to have had feeding issues with the magpul PTS green label line of mags
-Just to note, systema PTW mags need to be taken apart and cleaned once a season because they are well known to accumulate dirt and start jamming up.
For a beginner.... well there's one golden rule about mechboxes; don't fix what aint broke. Although under ideal conditions a mechbox (even in perfect working order) should be regreased at the beginning of every season, some mechboxes last 2+ years without being serviced without problems.
It's really up to you if you want to wait until you have an issue or have it maintained regularly. Honestly I just wait till my guns develop problems.
As for going in there yourself, well, some people just plain suck at anything mechanical. Up to you if you feel confident enough to do it yourself. I recommend taking it to a gundoc to learn how to do it, but it's easy to make a small mistake that prevents the gun from working.
Airsoft motors don't really require any sort of preventative maintenance.... They typically get replaced before the brushes ever wear out. But it's usually more cost effective to just buy a high quality motor than get a stock one rebuilt (when changing brushes it's highly recommended you also true the commutator).
Loose stocks, flimsy handguards, chipping paint and all that. Honestly there's a ridiculous amount of different models to cover, so I'll just say ask your local players. As for chipped paint, gun wear makes it look like you actually play airsoft lol
Lesson #6 - Breakdowns happen
Fact of life, shit breaks down.
Whether it be your own fault or someone else, chances are pretty good that at some point your gun is going to break down.
Also up to you if you want to fix it yourself or get someone experienced to do it. At the very least they can recommend what parts you should buy.
So to summarize;
-Buy a good battery and charger
-Buy good or high quality ammo
-Buy mags that work with your gun
-Don't keep shooting when you have a jam
-Clean the barrel
-Don't lube anything that doesn't need to be lubed
-All guns break down eventually
-Google is your friend
-And don't be afraid to ask local players for help!