1. Learn GIT basics
2. Fork and Clone the opensource project repository from github (or wherever they are hosting it)
3. Make changes that respond to issues on their bug tracker, or add features you think should be part of the project
4. Commit and push to your fork, and put in a pull request to the admin of the project.
5. Sit back and be proud.
6. Profit from your new kudos.

Extending @indelible, @seflorentino and @fibric answers:

• This shared link by @prank7 is great to learn the Github workflow.
• Another shared link by @hisabimbola shows the goods of contributing to open-source projects.
• Also, this answer by @oded on how to find open-source projects to contribute is awesome.

"how to get started?" - you normally start to use a piece of software and if it's open-source and you find a bug to fix or space to improve the code, you do it.

I’m working on a project to help beginners get started on contributing open source projects. github.com/Roshanjossey/first-contributions It’d be awesome if you could check it out and gimme feedback