Lots of projects could use help just working on bugs and small improvements. You are unlikely to get paid, maybe if your lucky some randos will tip you some sats. Nonetheless it's a small commitment and it's a great way to get involved and build some familiarity with developing for nostr.

Just seek out the client you use or one you're interested in, usually on GitHub, and look through issues and contributing instructions.

Reach out to devs, their almost always extremely welcoming and excited to help onboard a contributor.