Not sure how to start with it and which projects to pick. I am particularly want to work on React JS ecosystem. Anyone who can guide me how should I go with it?
A lot of companies say they do. Very few companies actually do. If you want to improve your job prospects and you have 1000 hours to spend coding in your spare time, you'd do better putting that time into learning specific things businesses want and building a portfolio of Github repos that demonstrate them, than making open source contributions even if the OSS contributions are relevant to the same problems.
That's absolutely not a reason to make OSS contributions. It's worthwhile, fun, and if you're a bit lucky can lead to a good job. It just isn't the optimal way to improve your career.
When you finished with the task create a PR.