Phil, following is a link to the MTH instructions for this locomotive:
https://mthtrains.com/sites/de...ction/20dl17452i.pdf
On page 42 they show the bottom of the locomotive, indicating the location of the body and motor screws. If you are as inexperienced as you say, I would not start disassembling the body and motor. Just add a tiny drop of oil to each axle point and the bushing between the drive wheels and the gear body, as well as a dot to each end of the pickup roller. As long as the locomotive is operating smoothly and quietly, leave it alone.