find a medium to handle the payment, ie; paypal,
decide how you want your registration process to work,
code it up, (there are lots of sites filled w/ tutorials on how to create user sections if you don't know how)
ie; reg form once successfully filled -> forward to paypal via paypal button -> admin receives email from form and checks if payment was successful -> once payment confirmed, create a link or some sort of admin panel feature to allow the admin to activate the account -> send email notification to registeree of successful registration.
Your question is so bloody vague, how about searching and researching this stuff before hand? It's not like you're not the first person to do it.
