Getting Started
Obviously getting to this very page is THE most IMPORTANT starting step you can ever take. Give yourself a cheer cuz you did good. This very site holds already tons of good info that will come in handy for you. Go trough as many pages as you can and use it afterwards to check for specific info.
After you did all that, here's some more info-sources you might want to strawl trough.
One of the most important resources is Mikes Flight Deck This should definately be the first place to go.
On FScockpit.com you will find a large (50+ pages) online article, containing over 650 links about simulator cockpit development. The objective is to give an overview. As a starter examples are given of simulator cockpits, to give a feeling of what this is all about. Followed by a basic introduction to a simulator cockpit architecture. Following this introduction, in sequential order the various possible components of a simulator cockpit are discussed. In order to give a broader impression of the simulator cockpit 'world' an overview of simulator cockpit suppliers and the simulator cockpit community is given.
Then there is the "how-to" section on flightsim.com. Look in particular for Roland vanRoy's DIY cockpit write up and the series titled "Building Kev's Cockpit".
You can find Dimensions on this very wiki site.
Inspiration from other builders
What keeps most of us building is when we see others and their progress. Robert Prather has a webpage called the "Hybrid FlightSim Builder's Registry": http://www.hsbregistry.com/
Get inspiration for your project from other folks who registered there.
Start planning
Here's a guide which might help you to start planning.