First of all, welcome to the World’s Greatest Hobby!
Being in it for over 35 years, I learned a lot of it the hard way, as I’m sure many fellow modellers here did.
It’s been already said, but I want to re-emphasize - it’s important! There are quite a few ways to cut corners on layout building. Here’s one I’ve used; if there is any home building going on in your area, see if the contractor (or foreman, whatever) is willing to part with remnant wood pieces. They will throw them out anyway, and will usually give them to you. Also, if you live in the middle or eastern US, Pink or Blue styrofoam is used alot for insulation. It makes great terrain, is easy to work with, and doesn’t make your layout heavier than it needs to be.
PLEASE DON’T skimp on locomotives!!! Especially in N scale. The difference in performance between good and junk is vast. If you can’t afford it yet, like the bright 14 year old said, save up for it - you will certainly thank yourself later! Atlas and Life-Like/Walthers Proto Series locomotives are relatively in-expensive, and are reliable, smooth runners.
Don’t skimp on track either. Will also bite you hard with poor electrical connection and derailments. Atlas track is not too expensive, and performs satisfatorily (in my and my friend’s experiences). If you want to use a type of track that has ballast moulded in, you can’t go wrong with Kato Unitrack. It’s pricey, but performs well.
As I remember my start, a kid mowing lawns, I waited untill I could buy quality. The times I got impatient because I wanted to just get something running fast - well, those things have been a part of the local landfill for years!
Welcome, and you have friends here!