Look at some of the open issues and identify one that sparks your interest.
If you want to work on the issue, leave a comment on it saying that you're working on it.
Then, create a new branch by typing
git checkout -b BRANCHNAME. Replace BRANCHNAME with what you want to name the branch.
Conventionally, you should use the issue number in your branch name. For example, if you decided to work on issue https://github.com/codebuddies/codebuddies/issues/491, you should type
git checkout -b issue-491 to create a branch named