Whole lotta bad info here, try heading over to
http://bodybuilding.com forums if you want things from people who really know what they're talking about. (edit: there's also some very good responses, I was mostly basing what I read on the first page)
If you're starting out and want to build muscle and size, eat heavy and supplement whatever protein you're not getting from diet with a powder. I use Optimum 100% Whey and BSN Syntha-6 as a meal replacement.
If you're cutting (trying to lose fat without losing muscle), make sure you're getting enough protein (generally accepted .75-1.0 g protein/Lb of bodyfat) and eat at a caloric deficit. Lift to preserve mass.
Once you're past the basic stages and have a good workout routine/good body image, creatine (I use Optimum micronized creatine) is a great basic supplement. I also take USP Labs Jack3d, which is a pre-workout booster (basically, tons of caffeine, 1,3 dimethylamylamine, some creatine and a bunch of other crap that makes you want to punch walls and blast music)
Please, for the love of all things good, don't try prohormones. You're not at a plateau, you're not a competitive lifter and its just not worth it.
I started lifting seriously about 2 years ago and went from 6'1/230lbs/20% bf with a good muscle base from years of sports lifting but too much bodyfat to 6'2/180lbs/11% bf with higher lifts and way better definition. It changes your life, but I did the first year and a half without anything but protein so don't go overboard with the supps.