Like in the command line you need to pull the current state of the repository once in a while. In GitHub Desktop this process is called by the
sync Button at the top right corner.
When you made local changes and want to push them you make a commit by writing something into the summary textbox. Then you press
Commit to YOURCURRENTBRANCH. Now you´ll need to press the sync button and your push is made.
Note: You can use emoticons, mentions and referals to other commits or issues directly from the textbox.
So the Sync button can be used to