Pick a ticket from the available tickets in the top-left of the sprint dashboard page at:
Make sure it does not depend on a ticket that is incomplete.
If you have any questions about the ticket, ask on the group chat.
Do the ticket. See our HOWTO for git. Sprint specific git HOWTO for devs with push rights: whatever works for you :-) If you don't like pull requests, don't send them. Commit and push directly to the main repo. If you do like pull requests, fork the main repo and send pull requests, but merge them right after. My favourite way is to work on the main repo, but create local branches (without pushing them to the main repo). Merge branches locally to the master and push to the main repo.