This command will generate new private and publik key pair.
$> ssh-keygen -t rsa
If you already have one, just open ~/.ssh/id_rsa.pub (location depends on OS)
$> cat ~/.ssh/id_rsa.pub
copy all the contents. open http://www.bitbucket.org/account/user/your-username/ssh-keys/. It should be under Manage Account > SSH Keys
click on Add Key, then paste your public key details.
remember to use the "SSH" version before cloning. or if you're already using HTTPS, just edit git/config then replace the URL.
$> vim .git/config