Bad news first: if you want to develop RTAS plugins, you're pretty much out of luck. Pro Tools is the only major DAW that doesn't have an open plugin SDK; you have to apply to Avid directly and get approved, which, while free, is very unlikely to happen.
Now the good news: the VST SDK is free and open, and if you're willing to shell out a bit of money for FXPansion's VST to RTAS wrapper, you can use your VSTs in Pro Tools. That's how I'm doing it.
Xcode ist just an IDE (Integrated Development Environment), so if you're on a Mac you'll probably end up using it for your coding no matter what SDK you're programming with.
Lastly, most SDKs I know of are written in C or C++, so make sure to brush up on your programming skills.