Are you familiar with web programming language like PHP?
Are you able to create html form and post data to another page?
Are you able to create database and store data to it?
If not, you need to grab some books and start reading them.
If you are confident to build a dynamic site using PHP, MySQL but not for PayPal only. Download the PayPal's documentations and read them. Understand the payment procedure and IPN first. While you are testing, do not forget that PayPal provides testing website for you to test the payment process.
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