Open Source is code made public for others to freely use and make. This brings together a community to help each other learn and grow together.
In order to help the open source community, I would recommend finding github repositories made to help you. This one was created byin order to help others learn web components.
In order to start contributing you need to take that repo and edit it. This can be done by forking, cloning, and locally installing it. For this repository, you need to run it with yarn instead of npm, which the readme explains how to do. Make sure that you install it first. Then you can run yarn in any element.
Now that you've found a repo and have it locally, you can edit it. Make the element better somehow and push it back to the original branch. This is what contributing is - congrats!