Registration form question

I want to add a custom field (education) in the registration process for my multisite network.
For example some of my users may be: teachers, students, employers....
1) What file i have to edit and what do i have to change?
2) I want to add one additional feature in Member plugin. The additional feature is the custom field "education". I want users to be able to filter the "search" button according username and education field too.
How can i do that?