How do I redirect users from the registration page to Paypal? Only users with a "Sign Up Code" are allowed to sign up to the website. If they provide a valid signup code, they will be directed to a sign up page where they can provide user information, and PayPal processing is used to process the fee. On completion of payment an account is created. 

How can I redirect user to Paypal?
How can I check the payment is paid by user?
How can I create the account if the user paid?
Use the Drupal Commerce module.


Add the Paypal payment method module; hook the login/register form to provide a different redirect page, or use the Rules module.
You can look at the generated transaction.
I would create the account anyway; set a flag, or something, for payed accounts.