Are you sure that you need a replacement? Starters usually fail because of warn brushes. A quick worn-brush diagnostic that works 80%, is to tap starter with a hammer. Several raps - strong but not destructive - will cause the stater to work for a limited time. Brush replacement can be accomplished by most mechanically-inclined.
If you must replace it, be sure to remove the grounding strap from the battery. It is the one with the black terminal - not red.