Go through the issue tracker for lemmy or lemmy-ui and look for some simple bug or minor feature that you care about. Then look for the relevant part of the code and try to fix it. You can also make a comment or post in the dev chat on matrix if you need help. Honestly there are so many issues which could be solved in less than an hour, especially in lemmy-ui. That way you can make Lemmy better and also get familiar with the code to make larger changes in the future.