I always wanted to do it because my Grandpa had one that got ruined in a flood at his house and he never got the chance to fix it up. Almost all the trains were thrown away be my father during the cleanup to spite my Grandpa (my father was a horrendous human being) and all my Grandpa was able to save was a cheap prewar set he had had since he was a kid and a caboose from the '50s.
Fast forward to my freshman year of high school. I had saved enough money to buy a cheap Lionel starter set and my mom helped me buy a 5x9 piece of plywood (that's what the shop I go to recommended for fastrack instead of 4x8) and one of those grass hobby mats they sell at hobby stores that looks real and you can shave roads into it. Over the last 6 years, I've added an elevated track, two sidings to the lower track (where the 50s caboose rests with a couple other boxcars), about 10 MTH buildings of varying size, I own 4 full sets, three engine sets (including one set of powered and dummy engines), a couple passenger sets, a bunch of cheap freight cars, and I've installed custom made shelves with track on it to display the engines I don't use regularly. I use the same room at my Grandpa's old house because my Aunt still lives there, and I don't have room for it at my apartment and didn't have it at my mom's apartment when I was living there.
All in all, it has cost quite a bit, a good chunk of the money I made from freshman year to senior year was used on it because I didn't have much of a life, the girls I did date didn't want expensive things from me, and I didn't have enough money to buy anything useful for my other hobby, cars. But it's not like I paid for all the train stuff all by myself (Christmas and birthday would get me a building or two and a boxcar or two from extended family), and the stuff I did buy was on the cheap end. It is not the nicest layout by any means, but I've made due with what I have, it looks pretty good, and I made it so I'm proud of it. It's also very peaceful and rewarding to put your frustration and pain in your regular life to make something good.
Tips for getting into Model Railroading:
Buy a starter set. This includes an engine and a couple of rolling stock/passen track and a base-level transformer. If buying Lionel, try and buy sets with CW-80 transformers to start out with because those will power almost any engine that will fit on an 0-36 curve.
Buy at the right time at the right places. eBay is your friend. Train shows are your friend. It helps a lot at train shows if you are good at bartering, and if you are not, bring someone with you who is. I once talked a guy down who wanted $300 for a postwar engine to $175. Also, find a good shop that doesn't try and bullshit you. The shop I use is incredibly fair. Also, as I said, buy at the right time. Lionel was once selling that powered and dummy engine set for around $600 but knocked it down to around $250, and I jumped at the opportunity because it's a beautiful engine. That is the most I've ever spent on an engine, so it's not like I'm spending a ton, and my next highest was the $175 one.
Ask questions. I never had anyone to teach me how to wire buildings so I just experimented and figured out what worked and what didn't. But it would have taken me a lot less time if I had swallowed my pride and asked someone how to do it. Also, look at other people's layouts on YouTube and stuff for inspiration. ericstrains is a great channel.
Do not restore anything unless it absolutely needs it or you plan on keeping it forever. Even a very well done restoration will get back less money than an all original train with a worn down paint job.
Know what you are working with. Always check to see if an engine or rolling stock will be able to work on the curves you run on your track, don't buy anything that might be to big for your layout, and make sure you have the right things to run it (like I don't think you can run higher end engines on the basic transformers with the orange handle).
Experiment with different designs so you can build a better layout in the future. This is important because if you don't have any experience with model railroading, you are guaranteed to fuck up at some point with the organization of the layout and the wiring. You might also change your mind on what you want the theme to be.
That's pretty much it. Everything else, you'll learn as you go along. As I said, you don't need much money to start out, especially if you have friends and family that will occasionally buy you a piece here and there. When I move to Florida in the next year or two and hopefully have more space to work with, I'll probably start over with the layout itself and plan it out better, and I'll probably change the theme to City instead of small town. I'll still use all the same pieces but my layout is not well planned at all and I didn't soundproof the tracks so it is kind of loud.
All in all, it is kind of expensive, especially to get started, but there are ways to buy cheaply and smartly and still make a good layout. Hell, I was a teenager from a lower middle class family and I bought a large chunk of it myself with money I made from working, and the rest were Christmas gifts and birthday gifts. It's not like I got stuff every month, I'd get those things here and there that I'd buy and then on Christmas and my birthday, Id get a couple things. We wwre never rolling in money but I was still able to build a good layout using what I had.