I found it a bit confusing at first but after a lot of reading, I installed the PayPal Website Payments Standard.
Searching for more info on this forum using that string will help. For my test account, PP handles major credit cards/debit cards as well as the standard PP payments.
I use this link to setup a developer (sandbox) account: https://www.x.com/index.jspa (the PP developer site is called X - how new age).
Once registered, I went down the menu to TEST, then logged in there and created a test merchant account, a test user account.
Once my merchant account was validated, I used the API data provided to setup OSC with that account data. I found I spent a lot of time on the sandbox client, but it was worth the effort.
There are other tutorials on how to setup the link back and other params on the sandbox account, but the developer site is a great place to start and there is a lot of information on that site.
Be sure you check over the order confirmations, order updates, order status etc when you get it to work. The fun really starts here, in customizing your store.