Best advice: Gear up a bit. It doesn't have to be fancy. S&S makes some pimp lightweight jackets (I was trying them on today, they are super comfortable and look good) for a good price. One in particular has a white body with black sleeves, picking it up tomarrow (had to order my size).
Ditch those worn jeans, man. I love mine, but I've had enough rash to cure me of THAT particular habit. First thing to hit the black stuff is your knees on these things. Check out Icon's pants, they have some decent cuts that will at least hold the asphalt out a little. I club in my Icon jeans, and get compliments on them from time to time.
As far as bike upgrades, FIRST thing is a pair of frame sliders. These plastic bikes don't take much to ruin. Second, seriously consider a gel seat. I run from here to Ocean City on mine (3 hours?), the gel seat helps a bit. Plus, it stops the bike from vibrating your sack to death. Aside from that, outfit to taste.
If you plan on running hard, especially in the city, upgrade to stainless brake lines. Helps your stopping a bit by eliminating the rubber hosing, which swells under hard braking, reducing effective braking power.
As far as places to get gear.... New Enough has been good to me. E-Bay rocks too, I find good stuff there from people who bag their bikes and get scared, then just want to offload their equipment. Kneedraggers is also a decent site.
About E-Bay: Careful ordering stuff from there. A lot of it (especially the carbon fiber) is paper thin and, well, quite frankly it's trash. Make sure whatever they're selling has a name brand on it. If you have a little cash to shell out, now's the time to hit it, because some bike shops are closing due to economy, and you can pick up 2007-2008 model riding gear for half price lots of time from wholesalers.
Aside from that, welcome to the addiction bro.