Maybe you've just gone through the regular tutorial. It'll show you how to do jobs, lesson, upgrade, gacha, and battle. You really can't do anything but what they tell you to do in the tutorial... and you should level once I think. Once you're out of the tutorial, everything's like normal.
You really can't mess up unless you accidentally get rid of an idol you want to keep... or unless you set your stat points wrong, as you can't go back and change them.
Also, the server gives you a few browser cookies when you authenticate yourself - if you keep the cookies, you'll auto-logon again. If you want the logon prompt again, delete all your sp.pf.mbga.jp and sp.mbga.jp cookies.
Also, for the luls: Gatcha simulator.
http://mugenmasakazu.plala.jp/cgi-bin/nijiuraIDOLMASTER/mogemaskillyourself.cgiRemember, you might spend hundreds of dollars on Platinum Gatcha, but your memories are priceless.