For an explanation of issue lables see: Issue Labels.
2. Claim that issue!
If you want to work on the issue, leave a comment on it saying that you're working on it.
3. Create a new branch!
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.