So i was just wondering what is the problem you are having with it, i have made a few store sights and know how it works.
1. do you have a merchants account/ a pro account, if not just call them and they will walk you through it.
2. once that is done you will have to go to paypal.sandbox and get an account, then make a seller/buy fake account, put in the test api info into what ever module you use, make a few test buys, test ipn stuff like that.
3. get the real api, which is in your normal account, under profile some where, you will have to make one and then get it.
and then switch it out for your test info, if you can't find a module for it you can hand put it in, which the info for that is in the sandbox site.
also you will need a ssl certificate, a access file, and set up the ftp server to transfer you over, you might also have to enable a few things in php.
but i am sure there is a module for what ever sight this is, drupal webpress, usually there will be a free one if not just buy one, or get an opencart/ubercart sight that you like to that has it easly.
email me if you need any help or questions, i might be willing to do it if you let me do alpha.
here is a sight i have made https://shop.diytechshop.com