You need to make some basic decisions about the era ( time) you want to model, model a real road area or make up a fictious railroad. What features do you want like industries, mountains, lakes, residential or farm areas or wilderness.? What is your budget for pre-assembled structures, rolling stock and accessories? What are your skills for painting and building benchwork or models?
How ambitious are yu to build your railroad in sections or tackle a large layout?
Much will depend on the house you get for space and available configuration.
Make a list of the things you gotta have, the things you wanta have soon and things that would be nice to have. Write your list in pencil and have a big eraser handy,
Buy ONLY things that are on your gotta have list until later.
Good luck. Remember: this advice is worth exactly what you paid for it!