Alright I'll help you out since at least you ask some decent questions.
First go read the FAQ section however as for the questions you have not answered in the FAQ's:
1) Up to you. But joining in a game is as easy as just posting "in" on a game day thread (but don't flake (unless it's a good reason or within a reasonable time to inform them (ie. 48 or 72 hours in advance should be fine) cause organizers don't like that). You can always find a team later, and see what different teams group dynamics are. Some people enjoy being hardcore WWII reenactors, others modern military, and even others as a "weekend warrior" type of thing. I personally fall under the slightly milsim/woodsball/recreational type of play.
2) Learn as much as you can from the vets but there are some things you just have to figure out yourself. Don't however, ask for advice and completely disregard it. Think it through and if it makes sense then it's all good, if not then at least have a compelling or well thought out argument against such advice.
3) "Ranks" are just internal. Some may use them to separate admins/regular players (ie. Admin/Mod/Member/Probationary Member) or more experienced and less experienced players (ie. Maj(President)/Capt(Vice President)/Lt.(Moderator)/Sgt(Senior Member)/Cpl(Junior Member)/Pte(New Member)) but at the end of the day they're pretty much "imaginary", we all just want a cold beer and some wings or whatever at the end of the day(although, if you do that then airsoft may not actually be "exercise" per se since you're putting those calories burned back on). This is basically like roleplaying, not the real army or anything (although some teams may treat it as such). Remember, at the end of the day it's all about fun, whether that's following hardcore strict military simulation (ie. a "Busmans holiday") or more of a relaxed style of gameplay to get some exercise and relieve stress from the working week.
4) Flat out, no. If you want to get into that role I highly suggest getting an AEG capable of full auto and making it a DMR (and playing single shot) then moving up to sniping. As well, the investment for a sniper rifle is high, about $1000+ to upgrade it to the "perfect airsoft sniper rifle" and even then it's not terribly much better than a regular AEG since after all they both operate using the same ammo, same velocities (unless the admins allow you to run hotter for sniper rifles but even then the ideal speed is still ~400 FPS on .20's so you can only really upgrade parts that add accuracy and consistency to your shots).
PS: Never use the word sniper to refer to the gun, call it a sniper rifle.
5) Either or but indoor might be more preferable since it may be closer (depends where you live though, ie. Downtown CBD vs slightly on the outskirts of town). As well there's generally less gear requirements for indoor (ie. you can wear jeans + t-shirt and won't be at a disadvantage compared to wearing the same at an outdoor game but remember always that proper footwear is important). But don't let that discourage you from outdoor games if that's all that's available, I started on outdoor games and they're fun in their own respect as well.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE