It's kind of tricky, but if you can bear with me I'll list the steps for you. Before you start, jot down the name of the admin account with which you don't have the password for.
Boot into Single User mode as suggested by dimmer by holding down the command+s keys during boot. You'll pop into the command line. Once there, type in the following:
1) /sbin/mount -wu /
2) /sbin/SystemStarter
3) niutil -list . /users
After step 3, you'll get a list of users. The admin account you're looking for will have a UID of 501+. If it's the only account on there, it's probably 501; but the UID doesn't really matter, it's the shortname you're after. I just wanted to give you a point of reference for when scrolling through the list. FYI, if the admin's name was John Smith, the short name is usually something like johnsmith, so look for something similar to the admin's name in the 500 range.
Once you have the shortname, you just type: passwd shortname
Where shortname is obviously the shortname you just grabbed. It'll ask you for the new password, and then retype the password. When you're done with that, type "shutodown -r" to restart your computer.
Once you're back at the login screen, select the admin account you didn't have a password for before, and use the new password you just gave it. Once you've logged in, you can create your own admin account, log into that one, and then delete the old one.
You should be good to go, and congratulations on pwning your first computer.
