Start by asking your Architect or Engineer for refferals.
Decide on 3 Contractors and ask them to Quote.
Once you have otained there Quotations, regarless of the price, request an indvidual meeting with each Contractor.
Set aside the time to go through their quotation, ask enough questions and get to know the Contractor.
The cheapest quote is not always the answer.
Once you have made a decision, ask to see his last project, if possible, and ask for refrences.
If you follow these basic steps, you should be in good hands.