Next register an account on bugzilla. Bugzilla can be daunting, so check out http://mzl.la/bugzilla_for_humans (15m video HIGHLY recommended).
Now look for a WebDev mentored bug in httphttps://mzlwiki.lamozilla.org/Webdev/webdev_mentored_bugs GetInvolved and contact the mentor - over IRC, email, or comment on the bug.
Finally, with your mentor, fork the github repository for the bug's website and you're ready to start coding!