Okay, so this is more from a learning standpoint. Being new to PHP, you should really stick with pre-existing software and learn to build on top of it, otherwise, you are jumping into a position that really requires much more experience (you will be better off evaluating the code of existing software and learning from it).
I've personally never created a free classified ads site, so I can't speak to that one, nor a social networking site (I tend to have clients go directly to those mechanisms rather than taking on those fields, they require too much effort from my standpoint).
As for e-commerce, oscommerce, is a good learning spot and so is zencart. Download them, install them, riffle through the code, force yourself to have to add "features", however, start with just styling them differently (changing the layout, etc).
The biggest issue you are going to have (since you have zero clients), is that you need to be the client and the developer. You need to create features/a business need and then full-fill that as a developer (very hard to do). Trying to do something without requirements will end in disaster, so you need to create a list of requirements first, then work towards accomplishing each requirement at a time.
As for creating payment modules, that is usually fairly easy, but the big part you need to learn is interacting with SSL on both a partial basis or an entire site basis (some clients only want the checkout to be covered by SSL, others, may want the entire site covered by SSL). Paypal has an API and developer documentation to help you out. So does 2Checkout, Authorize.net, etc. Most of the time, you just have to get the documentation and implement it (search google for examples when necessary or if you are unclear as to how to proceed).
I wish you the best of luck, but if I were you, I would start VERY small. Go to clientele that need a simple website and want the ability to be able to update the website (thus a need to involve PHP), or they want a specific feature (a calculator that calculates the monthly payment for a purchase on a home, boat, car, etc -- again something that could use PHP). By doing these small projects you will start to gain confidence in taking larger projects on and you will learn your coding style and architecture (both equally important).