Hi Michael;
I, too, am surprised at the lack of responses to your question. While certainly not a steam guru, or even a diesel guru (or any kind of guru, for that matter), there are a few simple points to keep in mind.
First off, congratulations for realizing that you do need to take care of your equipment! You're already ahead of the game!
Model steam locomotives, like model diesels, have a motor and a drive train. The hardest part of maintenance is taking the unit apart, and remembering how to put it back together again, without having any screws left over. Check the instruction sheet that came with your model. While not often the most helpful, they will usually have an exploded view that will help you visualize the assembly.
Get a load of cotton swabs, and some paper towels. Use the swabs to wipe off any old oil or grease.
The motor has bearings, which are at either end, and are where the shaft sits. Lubricate these with a good quality, PLASTIC COMPATIBLE oil, ONE drop ONLY!
The drive train can be of many different types. There can be a worm gear driving a single geared axle, there can be a drive belt connected to a set of geared axles, and so on. The key here is to lube the gears with, again, good quality, PLASTIC COMPATIBLE grease, and only very lightly smeared.
Often the axles will have their own bearings, usually sintered bronze or brass or something. You can also VERY lightly oil these.
The most important point (if you haven't already figured it out) is do NOT over-lubricate; this can be just as harmful as no lubrication (awaiting some fresh remarks here...;-)
There have been many articles in the hobby mags about this; by all means, look them up, or, if you don't have them, ask a friend. They will be most helpful. I will try and find some articles for you to look up.
Best of luck!
Jim