How you plan and design your layout depends on what's commonly referred to in model railroad language as "Givens and Druthers." The givens will be things like the size and description of your available space, the locations of doors, windows and lights, electrical outlet locations, etc. The druthers will be things like your maximum / minimum curve, the type of equipment you want to run, whether you just want to watch trains run or want to operate more like a real railroad, etc.
Spend some time looking around this and other model railroad forums (I know, it should be fora) for ideas to construct your own givens and druthers, draw a rough plan (either on paper or on a computer program like SCARM), and then ask for comments and concerns with your plan. Feedback will then be much easier for us.
Chuck