I'm certainly no expert, but I can definitely point you into the right places to start.
First and foremost, learn about the tools you'll be using. The popular IDE (Integrated Development Environment) program is Eclipse. It's much more detailed and in-depth than programs such as Notepad++ and makes Error tracking several times easier! Not only that, but Eclipse has an Auto-Compile feature that compiles your code changes as soon as you save the document, which is super handy! Most sources you'll come across are written in Java, so having a bit of knowledge of the Language certainly helps as well as it makes debugging a little easier.
You can find and Download Eclipse for free Here
After that you'll need a source, which there is no shortage of in the RS2 Download section. The package will come with 2 folders, A Source folder, and a Client folder. Both of these must be loaded into Eclipse by selecting File > New > Java Project, Unchecking the Default Folder checkbox and selecting your Source and Client Folders. Once you've done that you're good to go. It's all about surfing through labeled packages to see what does X thing to X object and so forth.
Beginners tip: You'll find some vital information for your server in the Config/Configuration.java files located in both your Client and Server folders.