My recommendation (although I am not a programmer by trade, and the quality of my code readily reveals this) would be to start by writing some small tools that fix problems you encounter in your day-to-day. Having a catalog of existing repos will give you much more credibility as an unknown and it will give you an opportunity to develop your documentation and code hygiene skills.

Once you have some nice repos under your belt, start looking at issues within repos you are interested in and give it a crack. Good luck and thanks for your interest in helping FOSS!