Congratulations on your new (old) house and your willingness to redo your front door. This is a good place to start building experience in self home improvement.
My suggestion to you is make a trip to your local library and look through the home improvement selection of books, particularly on painting. Time-Life, Readers Digest, Black and Decker and Tauton press have many how-to books with very nice illustrations and good instructions.
No, you won't need to strip the old paint off the door, as long as it is attached to the door well. You will have to do quite a bit of sanding to get the scratches out, and whereever you sand to metal, you will have to put on a primer. But you will have to do considerable hand sanding of the entire door and all molded edges (if any). Remember, 80 to 90% of a professional looking paint job is preparation.