There are many ways to contribute to open source projects: improving documentation, submitting accurate and detailed bug reports, helping users in the community, as well as submitting code and test cases.
If you want to code, then probably your first step is to *deploy* Prometheus in your environment, understand its philosophy, find some limitation that you come across, discuss it with the community, and work on that - in other words, "scratch your own itch".
If you're working on the Prometheus core, this is written in the Go language, so you'd need to get to grips with that. However, Prometheus includes a wider ecosystem of components including clients, exporters and remote databases, and these are written in a wide variety of languages. Maybe you'll want to work on one of those, or create a completely new client or exporter for a use case which isn't catered for yet. I'd say that's a really good place to start.
Good luck!