Contributing to open source happens at all levels, across projects and design, documentation, operations and code. You don't need to overthink what exactly your first contribution will be, or how it will look
Start by thinking about the projects you already use or want to use. The projects you'll actively contribute to are the ones you find yourself coming back to.
Check out these sites to find a project looking for your help:
Find a project that has a history and reputation for being welcoming to new contributors in GitHub's great for new contributors showcase.