administrator account became standard- no other administrator account

Can anyone help me?

I accidentally changed my "administrator" account to "standard" and I don't have an administrator account. I can't install applications now. Before you ask, this is a used computer and I don't know any of other administrator accounts.

I have a late 2009 iMac. The processor is 3.06GHz Intel Core 2 Duo and the memory is 16GB 1067MHz DDR3. This is my first Mac, so please be as kind and as clear as possible. I am definitely lost on this and need detailed help.

Thanks so much for anyone who can help.