First things first. Sit down and design the project. Locate existing electrical, plumbing and HVAC, take measurements and draw a scaled plan. Generally, build floors (you may be skipping this if you are going on top of concrete, or you may want floors to facilitate drains, plumbing and other infrastruction. Once floor is laid out, frame walls, including windows and entries, add electrical, plumbing and HVAC. Install drywall, then any fixtures and finish.
But the plans are first. From that you can set a budget, bid materials and labor if necessary, and besides, paper mistakes are easy to fix. Keep in touch as things move along.