How can I unlock my administration account ? or How can I completely erase file accounts and administration accounts from my apple computer?

So recently my sisters and my mother bought this used computer called a "IMac's IMac" . It cost $700 . This computer is great the only problem is ... There's a LOCK on the administrator's account . And we need the password so we can download "adobe flash player 11" without adobe there's no watching videos , playing online games , or even me editing photos for my photography class. If somebody could help me and my family with this. It would be really grateful !

We bought this beautiful computer on March 17th of 2012

Oh and i'm only 13

More Info:

Mac OS X

Version 10.7.3

Processor: 2 GHz Intel Core 2 Duo

Memory: 3 GB 667 MHz DDR2 SDRAM

20 inch, Mid 2007

Model Identifier: iMac7 , 1