Accept online payment and redirect users back to site

I cant seem to figure this out. The site I am building is for competitions. It will cost a user £1 to enter. All they have to do is enter their contact information and pay.
I cant seem to figure out how to handle the payments. The way I see it working is a user enters their information in an online form, they then click on buy and the user is redirected to an online payment site and then redirected back to my website where I confirm that payment was received and I then insert their details into the database.
Paypal buy now buttons don't seem to accommodate a redirect back to the site with confirmation of payment? Any help appreciated.