i need desperate help

ok i know im goin to sound like a dumbass, but me and my brother were setting up his computer and we never realized that we didnt put him as an administrator so he cnat make any changes to his computer b/c we dont know the administrator username or anything.!!! please do you guys know anything that could help me and my brother....we need help desperetly

here you go:

Mac OS X: Changing or resetting an account password