how to change my imac password? forgot my original one

i have forgotten my administrator password and i want to change it. the mac is version 10.6.8. when i login it doesn't ask me for a password but when i try to like update the software or install and update the new flash player it asks me for a password and i can proceed. i have recently gotten this mac from my mothers old job and i have asked them for a password and they don't know, they told me that i need to just press enter but i doesn't work for me. I have researched about this and the first article you needed know the original password to proceed or you had to restart the startup disk. does it work? if i restart the startup disk do i lose everything from the imac?