I’d suggest looking in the Contribute section of the site https://discourse.ubuntu.com/c/documentation/contribute
I know all flavors would love more people to come and offer. Testing and reporting bugs is easy, and a good place to get started probably the hardest bit (in my experience) is working out the package that contains the ‘bug’ & thus you end up learning some of the software stack indirectly (albeit superficially).
If you want to dig straight into coding & fixing problems, you could pick a package you use, and are interested in, and see if you can confirm existing bugs (ie. filed by someone else), then grab the source code and see if you can find the bug itself [in the code], and then add your ‘fix’ to the bug report.
If you’re familiar with packaging you could also help getting it out to others in the community via package updates (or just packaging it yourself with fix using a PPA is quick & easy, and useful anyway just to test your fix, or have others (or other boxes) repeat the test.
Pick an area that interests you, and ‘dive’ in is probably best. GNOME or the ‘main’ desktop probably has less options than the flavors; but this is my uneducated opinion.