In Mac OS X, how do I change my admin password if I don't know it?

I know I need the install discs. Can I use any install discs if I don’t have the original that came with the computer? I’m assuming yes. Anyone know if it has to be the same OS X version that is installed? Also- if there is another identity that have been using but don’t have the password to will I lose my access when I switch to the admin identity? Thanks! BTW- inherited a machine that was orphaned- nothing shady going on here- I just don’t have those passwords because the orginal owner has forgotten them. (Actually a cruel third party vendor didn’t provide them originally because they like to seperate people from their machines)