I've got a 33 year old 18 speed bike, the frame is in surprisingly decent condition. I am thinking about changing out the drive train though, the entire thing (front/rear cogs, front/rear derailluers, and the shifters. l basically wanna start it over from scratch but l do not yet understand the variances concerning the number of teeth in cogs or how higher or lower numbers can improve performance. Can you help me?

the parts from then and now have changed greatly and because of this you may find it hard to get parts for you bike cos the measurements and the fit are all different
you may want to look up shops that do vintage bikes cos they should be able to have old parts but with vintage parts also come the premium you have to pay

Quite honestly you are better leaving it as an 18 speed.
Parts would cost more than the bike would be worth. You can get a new bike for the same or less. And there would be 33 years of improvement. Shift indexing, much better brakes, better wheels, the list goes on.
The best thing for this bike would be to have a tune-up done. If you need to replace the rear cogset or chain, that is pretty cost effective and worthwhile.