Check out simple causes of leakage such as clogged gutters or obvious cracks around basement windows or doors. These can be easily addressed without the expense of hiring a waterproofing contractor.
Ask friends and family for recommendations of contractors they have used.
Check with the National Association of Waterproofing Contractors at (800) 245-NAWC about any contractor you are considering hiring.
Interview several contractors and have each inspect your house and prepare a proposal for the waterproofing work.
Compare warranties offered by the various contractors.
Negotiate a written contract with your chosen contractor.
Pay any deposit called for by the contract and have the contractor begin work.