Hi,
Try this to get you started:
1. On the Ribbon you can go to the Data tab and click on Form Load.
2. The Rules task pane will appear and then you create a rule the executes when the form is loaded.
3. In the Rules task pane look for 'Run these actions' and click on 'Add' next to it and select 'Set a filed's value'
4. For Field, select the field where the username should be
5. For Value, click on the fx button and look for the username() function
When the form loads the field for the user will be auto-populated.
I hope this helps, a good resource is https://www.youtube.com/user/qdabra/videos you may want to check it out :)
Jun