Well my friend, Wil, first let me welcome you to the forums, and second let me welcome you to the world of woodworking. This hobby can be kept very basic and inexpensive, but if you find that you really like it you can spend more money than you realize in a hartbeat.
Anyways on to your question. I would strongly suggest that you pick up a book at your local library on woodworking basics. There is so much to cover it is unreal and there is no way to cover everything in a single posting. I have a few articles over on my web site that might help you out a little as well.
I am assuming that you are going to be building a toy train and track system that they push around the track. I think I have a link on my site to a free plan for one of those as well. Depending on how complicated you want to make it I would start off with just a couple of tools.
and at a minumum a dremel type tool with router bits for that tool.
Of course there are some other tools out there that can make the job easier, but that means dropping more money.