As for where to buy stuff, check this thread right here on this very forum:
Best Hobby Shops on the Web
It is a long thread with a lot of recommendations.
i just want to know how you guys go about starting your layout
The first step in starting a layout is planning, not buying.
Buying stuff is actually the easy part. All it takes is money and a list of what you need. There are plenty of good vendors to choose from. See the thread above.
Figuring out what to buy is the hard part. That's where planning is important. It means having some idea of what you need for the layout you want to build.
Things to consider include what era you want to model. No point in buying modern diesel locomotives if you are modeling the 1940's.
Do you want to feature locomotives and rolling stock from a particular railroad or railroads? This will help determine the geographical setting for your railroad.
Or you can approach the other way around. Maybe you already know where your railroad will be located. If so, that may determine which railroad's equipment will be featured.
The type of track requires decisions, too. Do you want maximum realism or easiest setup, or something in between? Your answer will help determine which type and brands of track are best for you. There is no single answer that is best for everyone.
What part of model railroading most interests you? Some people like to operate a layout as realistically as possible. Others want to sit back and watch trains run through finely detailed scenery. Some enjoy the challenge of scratch-building each structure on their layout. Others purchase ready-made buildings but spend hours carefully detailing each locomotive.
Do you envision operating your railroad alone, or do you want a layout that will be operated by several people at the same time?
Are there some things that you absolutely must have on your layout? A turntable and roundhouse? A river? A large yard for making up trains? A coal mine? A farm? A city scene? Broad sweeping curves?
Make a list of these "must-haves" to guide your planning. These things are often referred to as the "givens" for your layout.
Make another list of things that you would like to have but aren't as important as the "givens." This secondary list is often called the "druthers."
With a list of "givens" and "druthers" you can start thinking about what is practical and impractical for you to accomplish. You can start to get an idea of how much space the layout will require and how much it is likely to cost. I think most model railroaders have faced the hard reality that there isn't space, time, or money to do it all.
Thinking about things like this should be your starting point for any layout, long before you worry about where to buy stuff.
- Jeff