Look at some of the open issues and identify one that sparks your interest.
If you are looking for issues that are good for beginners, check out the issues labeled "good-for-beginners" or "good first issue"
For an explanation of issue lables see: Issue Labels.
If you want to work on the issue, leave a comment on it saying that you're working on it.
Create a new branch by typing git checkout -b issue-x
, where x is the number associated with that number. For example, if you decided to work on issue #491, you would type git checkout -b issue-491
and create a branch named issue-491
.