On the Internet today, there are many people who call themselves web designers and web design companies. Some people do it for fun, as a hobby or part-time. Web designers that design full-time are the way to go.. Someone who does web design in their part-time tends not to take it as seriously as someone who does it for a living.
There are also people who do web design, who have no formal training. Make sure your web designer has some formal training from a recognized school. People who are self taught, although I applaud them for doing it, have relied on the Internet for all their information, and as you know information on the Internet can be inaccurate.
The web design firm you select to work with should have experience in the field. If you'd like to create an E-commerce site, it's recommended to select a designer with e-commerce sites within their portfolio. Your web design company must be able to provide an offer for your web project, along with the contract. Beware of businesses that do not offer an official contract.
The contract should specify precisely how the designer will accomplish and the time they will need to complete the task, as well as the cost, and the details you've agreed to provide.