Welcome. Introduce yourself at the Model Railroader sub-forum on this site. You'll get lots of help.
They'll want to know if you have selected a scale. Also, which era (early, early 1900's, transition and war, post WW II, modern). How much space do you have? Do you have a club or other help nearby?
Have you heard of digital command control? It's shortened to DCC. Locomotives need a microchip installed to control the DC motor so that power is full on the rails all the time. The decoder installed detects your command signals and makes the loco do what you want. You don't have to reverse the DC current to the rails any more.
That's just a short intro to get you thinking. Again, go to the scale trains forum. There are others around the www, too.
-Crandell