1) shoot it lots, buy ammo and get good with irons first
2) when you buy anything don't go cheap, especially with optics
3) Avoid, NCSTAR, TAPCO etc
4) You can buy accessories from any of the retailers listed at the top of the site here. I'd recommend Primary arms from my experience with them. PSA is good also (people complain about their ship times). Brownells and Midway are the Amazon's of the online gun world and are generally pretty decent too.
Finally work out what you want to do with your rifle before you spend $1000 on stuff you don't need. That way you won't end up with a tacticool nightmare that is heavy as shit and isn't good for any situation.
As far as optics go i'm a big fan of Eotechs but I know a lot of people prefer ACOGs or Aimpoints...both of the latter are truly excellent options.
Most of all read up here and on other forums and learn and remember it's all about having fun