If you're not sure sure, then maybe start by figure out an area of skill you want to improve on or just plain like doing. Then go and find some OSS projects that use that skill or could benefit from someone with that skill contributing.
If you happen to have artistic or documentation writing skills, there are heaps of major projects you could really help level up by creating a bunch of official guides and documentation for them.
So, what sort of stuff are you into and wouldn't mind spending time on?