There are plenty of reasons to contribute to open source projects, but it can be confusing how to get started.

Trying to fix a bug or implement a feature is something like the Linux kernel or React might sound lucrative, but is also probably extremely difficult. Most of the issues there are probably quite difficult.

Small projects

One way is to start is to find small projects, and "practice" on them. There you might be able to send a pull-request after a few minute or maybe a few hours of work.

Non-code to a bigger project

The other direction is to go with the well-known project, but start with the help channel. Start looking at the question other people have and try to see if you can answer them.
Read the open issues and try to help with the new issues. There are many case where people open a bug-report which is a duplicate of an existing report. If you can help the maintainer filter out these issues that is already a huge help and in the meantime you start to learn about the project.

Join the Open Source Development Community

I am trying to build a community of people who help each-other with the first steps contributing to open source project. You can read about the community or you can go ahead and join our Zulip server.

It is a chat server similar to Slack or Discord, but it is fully open source.