Starting point: Explore repos on github of your interest and use https://www.codetriage.com/ to get a handful of issues from your favorite repos in your inbox every day. This service will help to save your time searching for issues.
Later you can narrow down projects to contribute and go directly to issues list and pickup.
Also look for PR generated in the project you are interested. It will give you different viewpoints which is very important for building real engineering capability.