Workspace tab
Now that you have selected a repository and picked a branch you should have the workspace tab open.
At the top of the tab you have the details of the selected repository & branch and the add to favorites <3 button. Below that is the add file button which will create a new empty .md file in the root folder of the repository or create a new folder along with it.
Alternatively you can create a new .md file anywhere you want by simply right-clicking that folder and selecting 'add new file'. You can also use right-click to delete or rename files.
You can obviously also select an existing .md file and begin working in it using the editor.