Start by making a spread sheet of all the things that you know you will need. Assign a value in dollars to each one. Add them up. Then add the things that you forgot or didn't know about yet.
Rent or purchase a place? Renovate to your specifications for a store: electrical, plumbing, shelves, flooring, ceiling, HVAC, $100,000.
Inventory? $100,000.
First year's payroll (includes accounting fees, taxes, benefits, business license, etc) $50,000
Website, signage, print ads, flyers, postage, credit card set-up, inventory software, POS system, utilities, insurance, credit line... $100,000.
How does one become a dealer for the major manufacturers? There must be some minimum inventory that you need to commit to.
Do you have a quarter mil handy? If you do, probably a better place to put it is in T-bills and use the interest to buy yourself some trains. If you are planning to teach your parents a lesson, this is the wrong way to do it. Invite your friends over to help build the layout. That will help with social skills.
Like many other business ideas: "How do you make a small fortune selling (fill in the blank)?" "Start with a large fortune."