Building a successful web site is dependent upon many thing. Success, though, must be measured carefully. To do so, you must define your goals first. You must understand what you are trying to achieve before you can measure it.
For example, your goal might be to increase sales. The goal is easy but how do you measure it? You must define by how much you want sales to increase. Another goal might be to increase productivity by solving more customer inquiries online. You might evaluate this by comparing the decrease in calls to your customer service department to the increase in your online help requests.
Whatever you do, don't set out without defining your goals. Once you have done that then we can focus our expertise to your needs.