I found it somewhat difficult to get started with PayPal because x.com (developer portal) didn't give me the right guidance. Maybe I'm an idiot? In any case, here's a little guidance for you.
Start with the e-commerce videos on https://www.x.com/community/ppx/training. They are really helpful.
Then visit the PayPal Tools page and read about each product to see which one you need. There's also a tool to help you decide, and you can watch videos about them on the training and tutorials page.
At this point, you probably want to read the Apps 101 page. It explains how you make apps that use PayPal to process payments.
Finally, go to the main page for whichever product you're going to use, and start digging into the documentation and the SDK.
Other Helpful Notes
Some other things confused me, so here is what I've learned. Please correct me if I'm wrong and I'll keep this updated.
2 Kinds of PayPal Apps - Most of us are just using PayPal to process payments. But now PayPal has a new kind of app you can develop, and it's referred to as a PayPal App - confusing, right? This new kind runs on PayPal to augment PayPal's features.
Hope this helps!