Choose something from the list of existing bugs. You can filter by tools (e.g. only Console bugs), and also by good bugs for beginners.
Or if you would like to work on something that is not listed there, file a bug in Bugzilla (you’ll need the Bugzilla account you created earlier) and ask for it to be assigned to you. Please also try to initiate a conversation in the bug first, to ensure that you don’t work on something that will not be accepted (for example, if you think you found a bug, but the feature worked that way by design).