Depends really about how you play, what kind of area, what kind of guns you/your group uses, etc. As far as online retailers go, there's plenty of trusted shops out there such as Airsoft GI. (Airsplat, Airsoft Atlanta, and Evike to name a few others).
Most important thing above all else is good eye protection. Face protection in some areas is generally optional because it can interfere with the ability to aim with some guns, but good goggles (Eye protection) are a necessity. Protecting ones face can also be somewhat achieved with head wraps, balaclavas and the like.
BB's depend on what kind of gun you're using for the correct weight. If you're using lower powered spring or electric guns, you'll probably be looking for a bag of 0.20g bb's. I tend to go with Airsoft Elite, KSC, or whatever brand is having a good deal at the time. Generally I also prefer to order these online as they're usually a bit cheaper and of higher quality than something you'd find in a sporting goods store, though you do have to pay for shipping.
For a beginner, I also wouldn't really recommend any tactical gear or whatnot. It's perfectly fine to play in a tee shirt and jeans, but if this turns out to be something you're doing often, you can go from there as far as camo goes. Pouches and stuff aren't really necessary for everyday play, and if you happen to change weapons in the first few games, your M4 mag pouches might not fit new AK mags, and so on.
Anyway, hope that helps. Oh, and for boots, I personally didn't go for anything fancy. Went to Wal-Mart and got a pair of black waterproof boots that looked and felt OK. Been using them for well over a year in all kinds of weather (Rain, snow) and they've held up just fine.
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