what field should i choose to make it as primary key?

what field should i choose to make it as primary key?

what should i do if i have a login form on a web page & it prompts me to
enter an email as a username & password. if i design such web page should
i take that email as a primary key? Because i'm not giving user a field
during user registration so that he/she can enter any integer ID that can
be chosen as a primary key further. I also have other tables which belongs
to this user & i need join to retrieve all the details. In this case how
do i manage primary key. i can't take it auto generate because that needs
to be entered in other tables also at the same time,which can't be
possible with auto id.
Need help?