So you're looking at this screen?
Now if Ubuntu starts without a password one of two things is happening:
- You didn't set up a password. In this case just hit enter when it asks for your password and you're off to the races.
- You set up a password but told Ubuntu to automatically log you in. In that case you must remember your administrator password used when you first set up Ubuntu and type it in. Ubuntu passwords are case sensitive. Bob and bob are two entirely different words.