[vin17]

There are a lot of accessories that you could do without. However, a couple I would recommend are the controller charger and the wireless mic.

With the controller charger, you can continue to use the charging controller while it's connected to your 360. It doesn't take too long to charge. As well, unless you want to spend an 10 bucks for a pretty negligible difference in charging time, make sure it's the 'Play and Charge' kit. Also, I'm not sure if the other type allows you to play while it's charging like the P&C kit.
edit: ok, the other one is called the Quick Charge kit. It doesn't sound like it'll let you play while it's charging.

As for the wireless mic, if you're not a fan of wires then you might definitely want to consider it (and really, what's the point of a wireless controller without a wireless mic). Although they're 50-60 bucks, the sound quality is excellent and they hold their charge/charge up pretty good.

Having not played a FPS shouldn't be a problem. Each successive game in the franchise has been targeted at a more larger group of people, for better or worse. If you're just going to get Halo 3, you may wan't to brush up on the storyline otherwise you might be a little lost campaign-wise. As for playing with other new players, you might want to try the 'Basic Training' playlist to get your feet wet. As well, it has an unranked, more 'Social' type play list where most of the players are looking to have fun, not necessarily win.