OriginalGriff is absolutely correct regarding the fraud issue.
But if you like to research, then I would suggest you the popular
Code Project Article Introduction to PayPal for C# - ASP.NET developers[
^], which will guide you step by step.
Saying the above, I would also strongly recommend you to go through the codes carefully before implementing.
Otherwise you end up with transferring money to someone's account without your knowledge.
So, code safe and be safe. :)