Try using the split polygon tool to draw the outline of your door. Then use it again but this time draw the outline of the door offset from the first line. Then select the faces inbetween the two lines (all the way around the door) and then extrude them inward.

kinddoggofwar is modeling a vehicle in the WIP section and this is what he's doing. Check out his thread.

Although, I'd probably recommend trying to follow whatever tutorial your working from.

Also, save you project in a separate file first, that way if you mess up, you can revert back to your original file and try again.

Cheers!