The first step in contributing to an open source project is to fork it. This means to create a copy of the repository in your account. Once you do this, you can then make whatever changes you feel are appropriate. You can see which of your repositories are forks as they come with a note under the title (red added by me):

A pull request is a request from the forked repository back to the original repository to add the improvements. You can easily create a pull request from any branch back to the original repository, as they will have this banner above the code, and you can click Pull Request to start one

After doing this, you need to describe what your pull request does. Many large projects will have a template for pull requests, and the contents of that will be filled out already for you. You should edit this to describe your particular change, this allows the maintainers to better understand what your pull request does and why it is needed. If your problem is solving a certain issue, you should also mention that in your pull request.