Use the "Clone / New" button and "Add a Working Tree". Choose the folder on disk where you ran git init
. This will get your repository into SourceTree.
To push to Bitbucket, you'll need to add your remote repository URL. While your repository is open in SourceTree, click the Settings button and add a remote. Choose to make this the defualt remote repository with the checkbox and put the URL of your repository in the URL field (for example, "https://bitbucket.org/your-name/your-repository").
You can also do this second step from the Git Bash using the command:
git remote add origin https://bitbucket.org/your-name/your-repository
After this, you can commit and push your changes to Bitbucket.
If you wanted, you could also have created the repository directly in SourceTree by using the "Clone / New" button and choosing "Create a New Repository". This does the same thing as the git init
command and immediately puts the repository in SourceTree for you to use.