create custom fields

[PcG FithAce 7]

Hello. I have searched for days to try to find a plugin/extension or find out how to create custom fields/questions on my forum's registration. Apparently I havent looked in the right place somehow, but I know of course its possible because even like iclanwebsites lets you customize members entire registration. Im pretty new to this but I am trying to learn all I can on my own, however I'm to a point that I knew it was better to break down & ask a few questions. Can anyone give me a solution?

Thank you very much for your time & assistance.