For the very basics, you need to first look at videos that have been posted here by Benz Trainz both here and on youtube. Benjamin has a good way to clean track and remove rust. He also takes inoperable locomotives and makes them work again. After you watch about a dozen or so of his videos, you may want to embark on attempting to get everything up to being operable. Or, you may just opt for purchasing some trains that are currently in running condition.
Either way, you will gain some great insight and knowledge from Ben. Simple fixes at little or no cost, and they will be fun to do.
Since your original post was to seek knowledge, I know of no other place than with Ben's videos. Start there.
Bens videos are probably easier to find on YouTube then here. Here is a link to one for derusting track using a solution of white vinegar 5% solution.
Good luck, I think you will be pleased once you start watching Ben at work.