Since you don't mention your level of experience and/or what sorts of projects you have worked on and/or whether you have UI skills and in what framework, it's

Since you don't mention your level of experience and/or what sorts of projects you have worked on and/or whether you have UI skills and in what framework, it's going to be difficult for people to decide how much effort they will need to expend. As an earlier commenter hinted at, I would suggest trawling GitHub for any C/C++ projects that have tags matching one or more of your personal interests. Find projects that need support rather then just 'throwaway' projects and ask to get involved. Just my 2c.