How to find what to do
This passage is written to be general that can be also applied to other open source project. Many people use some program, and want to help with some project, but don't know what to do.
Find something that scratches your itch
The very first thing is, find something that scratches your itch. Fix bug that annoying you, implement features that you interest in. Especially for new feature, new feature have more possiblity to introduce bugs, thus why from project point of view, need some people to maintain the code in the future. Bugfixing is much more free, even if you work like "hit and run", nobody will complain about it.
Contact with developer
Secondly, ask developer about what do they think about it, even for bugfixing, it need to be fixed in the right place， due to Input method is relatively complex (If an application have complex internal state, it is complex).
If you wrote a long function but finally rejected by developer, it will make you feel quite bad, right?
Read the TODO-List
Todo-List is something that developers already have in the mind, just they still don't have time to implement yet, while usually it's filled with things that hard to do, but sometimes it will also have some trivial work, but maybe a little bit boring. Those are also work you can help with! It's human nature that people don't want to do some repeated non-creative work, but those are also necessary and will benefit the whole project.
Have fun with those open-source project! If you don't need to be under pressure that you have to finish something in some restricted time, just make yourself comfortable and have fun.
Well, actually, I have already seen lots of people show interested in some project, but also give up so quickly. It really doesn't matter if you do so, people have their own life and their own interest, we also respect that. No matter when you want to give up, just tell us in advance, so it would be more easier to rearrange the work and priority.