Choose a project you use daily. The most meaningful contributions come from real pain points, not from random 'good first issue' lists.
Read CONTRIBUTING, code style, and related issues before writing any code. Many PRs are rejected for repeating discussions that have already concluded.
Make small PRs with clear descriptions: the problem, the solution, how to test. Reviewers appreciate changes that are easy to understand in five minutes.
Suspend your ego. Review feedback is part of the process. Fast revisions build a better reputation than a perfect PR on the first attempt.