How do I get back to my own email account when someone else has logged in their's on my computer?

When my brother uses my computer he logs into his yahoo account. When he closes down and I reopen the computer next his account comes up not mine. I have a heck of a time trying to get back to my own email/AT%26amp;T webpage. Whats wrong? I try logging his account out and relogging into mine but sometimes get an error message or it will not get out of a general yahoo site (if that's the right term) HELP!
How do I get back to my own email account when someone else has logged in their's on my computer?
tell him to logg off before closing it out...or try creating different user accounts on your computer so you dont have to be sharing. thats what i do anyway =]