Look at some of the open issues and identify one that sparks your interest.
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