A good place to start would be with a project you use already. The best things to do to start are to look for a CONTRIBUTING.MD file in the repository and read it.
You might also need to sign an agreement (CLA), depending on the project.
Once you have got to grips with what is required in order to contribute, look at the issues and see what people are requesting.
If you already have something in mind, first consider if other people would benefit from it before opening a PR, otherwise pick something that seems useful and get started.