If you want better help, I'd suggest outlining what it is you know, and a better flow of your logic. What type of system are you looking at: mySQL, Postgre, MSSQL? Are you going to use a Flat-File Db, or any DB? What is it that you do to "activate" an account?
Answers to those questions would help us guide you in the right direction. If you're looking for a 1-on-1 helper for hire, check out www.scriptlance.com.
~Brett