First of all, WELCOME! Glad to have you here.
Second, for classifieds try here:
http://www.nebraskafirepower.com/forum/index.php?board=9.0Every once in a while you'll see a gun or 2 on Craigslist, but the ads get flagged pretty quickly.
What kind of rifle are you looking for? For your first rifle I would suggest a nice used semi-auto in .22 long rifle. Cheap to shoot and cheap to buy. You should be able to find something decent for under $100.
If you are new to guns, I would suggest
at least enrolling in a hunters safety course first before you buy a gun.