If you have an existing project you can synchronize it with your repository by setting the remote location and pushing it up.
If you don't have an existing project you can create one easily using the commands:
$ cd /path/to/my/local/project
$ git init
$ git add *
$ git commit -m "My initial commit message"
From the BitBucket page entitled "Push up an existing repository" that's shown when creating a new repo:
$ cd /path/to/my/local/repo
$ git remote add origin git@bitbucket.org:mike/myrepo.git
$ git push -u origin --all # pushes up the repo and its refs for the first time
$ git push -u origin --tags # pushes up any tags
After this point you can push
and pull
just as you would normally do.