I can't add too much to all the wisdom that's been given out here...maybe just to touch on a few things that have occured to me...
First of all, digest as much info as you can by reading, going to hobby shops, train shows, etc...
Although I had some train sets as a kid, I didn't get seriously hooked until about 10 years ago, & I can cite one thing that triggered my obsession with this great hobby...I bought an HO train set, & chanced to pick up a magazine that featured an article about George Sellios' Franklin & South Manchester, a spectacular HO layout.
Well, that did it...I was done for! That's what I've been striving for ever since. I've built layouts in HO, & N scale...I can't really say I prefer one over the other...they each have their own qualities...sort of like wine, or food.
Ok, I know I'm starting to ramble here...sorry...
Start out real simple...get yourself a starter set in a scale you like the looks of. I think HO & N scales are the best for beginers, because they have the most products commercially available. Get some books with track plans, & try to determine what you can do in the space you have available, & keep in mind, that bigger doesn't mean better. You can get a lot of enjoyment from a small layout, & you can always expand later.
I think the most important advice is to look at the work of model railroaders...books, magazines, the internet, clubs & shows...find something that grabs your imagination, & go for it!
Good Luck, & stop in here, & let us know how you're coming along.