1. Pick the project. Something that you like, but also something that is not huge or very mature (they tend to have longer turn around due to the volume of things they are getting). You can then check issue tracker for the project and see if there are tasks labeled "need help", "good first issue", etc. They are usually things that team would appreciate help with. On GH, it looks like this: https://github.com/iterative/dvc/labels/good%20first%20issue
If you don't know the project, try to search GH, may be here - https://github.com/explore ?
2. Check that project is "healthy" - check the recent history of PRs - that code is being reviewed and merged. It can be very demotivating to do stuff that won't be answered at all. Ask in the issue you like and see if there is a response.
3. Find the contributors guide. There should be a link in the project's README, something like https://github.com/iterative/dvc#contributing . The guide should help you understand how to do setup dev environment, how to create a PR, etc.
4. In a lot of cases these days, projects have chat or forum (or GH issue comments) to ask questions, ask help, etc. Check them our, see if they are "healthy" and don't hesitate to ping the team - usually people are responsive and are willing to help.
Good luck! and thank you for considering doing OSS contributions. That means a lot for them!
PS: it was an accident this year related to hacktoberfest :) but it is still good starting point