If you're new to open source (which everyone was once!), you can take a look at the Introduction to Open Source tutorial series.
To make your first contribution, it's required to familiarize yourself with How To Create a Pull Request.
The following resources share repositories that curate tasks for beginners:
Once you start feeling more comfortable, you can continue to find more open source projects that can use your help through the following programs:
- Pull Request Roulette
- CodeTriage
- 24 Pull Requests (Like Hacktoberfest, except in December)
Visit this guide to find out more about how you can contribute to open source.