Tips on using SourceTree with Git.

One option is to not use a branch at all, just work on the master repo.
This is a good option for just uploading a bulk operation.
In this case, just change your files and Stage the changes.
Then just commit the changes and push them.

Create a develop branch.

Check out the develop branch.
Make your code changes.
Your new code should be in the File Status tab, Working Copy Changes.
Review your changes and click the up arrow to push all of your code to Staged Changes
Click Commit and enter an appropriate note.
Your develop branch will now be above your master branch.
Click on master and check it out.  Will see the purple as your main line.
Click Merge and select the develop branch.  (Commit merge immediately).
Develop and master should be on the same line.
Now do a push of master up to the remote branch.