Your best bet is to decide what type of tank you would like to build. plywood, acrylic, plastic tub......
With a 200G tank, you might be better off just buying a glass or acrylic tank. Most people on this site build tanks much larger than 200G. Mostly because you cannot buy 500, 600, 1200 (whatever) gallon tanks. It might not be very cost efficient to build a 200G tank.
If you are still interested in building your own tank. Just browse throught he forum and look for the DIY tanks. Searching is always the best way.