1- Make a form where the user can enter their email adress.
2- If the email exists in the database, create a random md5 hash that you store in the user's table and send an email containing a link to reset.php?user=$username&hash=$hashhere
3- When the users clicks the link and go on the reset.php page, it verifies that the username has the right hash in the database.
4- If it meets the criteria, give a form to reset password. If not, show an error / 404 page / what ever you want.
5- Profit
No one is going to spoon feed you. Check up online how to make one and if you start coding it but get stuck, someone will help you fix it. No one will code it for you unless they're feeling very generous.