Two ways to do invunerability.
First way. Open the object editor. Click on the 'Units' tab. Find that type of unit. Double click on the field that says 'Abilities - Normal'. Click 'Add', and from the pull down list, pick 'Invunerable (nuetral hostile)' This will make ALL units of that type invunerable.
Second way. This is more for just ONE. Go to the trigger editor. Inside 'Map Initialization', add the action 'Unit - Make unit invunerable'.
Start locations? This is where the starting units are created; also it is where the persons camera will start. For example, a human player would get a town hall and five peons. If you don't want starting units, go to the trigger editor, and delete the triggers inside that create them.