I hate to say it, but if you have to ask you are probally not ready.
The specific answer to your question is file ariticles of orginization, or form an LLC in your state.
Then register the company with the state as a professional engineering company (i am assuming you are a PE).
Then get O&E insurance.
Make sure your owner resume is in tip top shape.
Hopefully you already have work, but if not get the word out with contacts you have formed thru the years.
Good luck
Tim
