It’s not so complicated. I have started to look into SDK code like one week ago from exactly same reason than you are thinking of doing it and my first PR was merged into master yesterday. Previously I haven’t worked in netbeans. Start with small details that aren’t hard to fix and eventually you’ll be able to contribute something bigger if you keep going 
You can get details on building the sdk here: [SOLVED] Need help with JME SDK building - #15 by Vortex
Hope this helps.