Mason - It's great to have you as part of this wonderful hobby! You have come to the right place for advice because so many members on this forum are truly wonderful folks willing to share their abundant knowledge of model trains ... and real life trains for that matter.
My advice for you, is to create a track plan. Once this is done, start out by purchasing whatever track you can comfortably afford. I say this in view of your being "broke as a joke", as you put it. Tubular track is the least expensive and might be your best way into the hobby at this point. When it comes to purchasing switches, used Lionel switches are pretty inexpensive, especially when buying at train shows. ( being 15 you may have to depend on others to get you to the show ) Although tubular tack does not present the most realistic look, once on the layout it can be "enhanced" to look pretty darn good .... i.e: ballasting, painting the rails, adding more cross ties. There are many members on this forum who use tubular track and their track work looks fabulous!! Take some time to browse ( I'm sure you are doing so already ) and check out various members photos who use tubular track.
If your space allows, I would recommend that you have at least 0-42 curves or wider. These wider curves will allow you to run longer/larger locomotives, and rolling stock, when your budget allows their purchase.
Remember too, that this hobby can be a life long journey. As your train budget increases you can replace your tubular ( if you wish ) with Gargraves, Ross, MTH Realtrax, Lionel Fasttrack, etc. It's your railroad and YOU call the shots.
Attending train shows is a great way to find some wonderful items a some truly low prices ( and sometimes not
.
As for passenger cars. All the manufactures you mentioned make very nice cars. Williams only has silhouettes in the windows with no interior. MTH cars, except for their earliest cars have full interiors. Lionel post war cars ( 1946- 1969 ) have silhouettes and the later ones have interiors. These cars, new, have quite a price tag, however, you can find great deals on them at train shows. If going with new, Williams packs the best bang for the buck .... if you like silhouettes ( I personally have both kinds and like silhouettes just fine ).
Lastly, I checked your profile and see you have a birthday coming up. Why not ask relatives and friends to give you train birthday gifts ... you can even give them your wish list. Just a thought!
I hope I've been helpful to you. I'm sure you will hear some great ideas from other members of this forum as well. Good luck!!
