Help us to help you.
Firstly, what code do you currently have? Could you post it somewhere (github’s ideal, but pastebin is a good second)?
Secondly, are you trying to do this for a real site, or to help you learn how to code?
If you are trying to solve a problem for a real site, then there are a lot (A LOT) of account management software solutions out there. Consider using one that already exists.
If it is to help you to learn then great – that’s what forums like this are here for, but asking how to write your whole site is a bit much. Show us what you already have written and what you know, and we can work with that.
To answer your question directly, the next steps after creating a registration form would be:
1) Write a script for that form to send information to that: a) assigns the information from the form to variables, b) validates the form data to make sure it’s acceptable (no empty usernames, please), c) safely inserts the data (and hashed passwords) into a database and checks that the insertion executed correctly (no duplicate usernames either), d) redirects to a login page OR sets the user’s session so they are logged in, +/- sending a confirmation email to the user to validate the account.
2) Write an account page so the user can edit their information including a script that basically does what 1) does but updating records rather than inserting them.
3) Write a site that users want to register for.