First, start with the basics.
Gain a general understanding of how a website should work for your business. Get basic information about web design, SEO, and SEM. Clearly define the goals of your website design, and how you can achieve them. If you don't know, ask someone. If you can't find someone, search on Google. The main goal here is to increase your general knowledge of how a website works, and how to make it work for you.
Meet with a professional web designer. Whether you decide to use them to build your website or not, they can offer helpful information about meeting the goals you have with your website. If you are going to use them for the build, take a good look at their customer service policy. Anyone with a 24-48 hour response time is likely to be a good bet. That way it eliminates most "email runarounds".
Look at the background of the webmaster. Where did they come from? What did they do before web design? Does that help meet your goals? There are thousands of people who build web sites out there, be sure to pick one that is right for you. There are college kids that can put something together for around $500, and web development firms that build sites over $50k. It is important that you find someone you are comfortable with, and would like to have a business relationship with.
Once you have found a webmaster you are happy with, ask questions. They are an expert at what they do, listen to their suggestions. Many times people shoot themselves in the foot by insisting on a particular design that actually hurts the overall goals for a site. There will always be a give and take between aesthetics, creativity and form/function. The key in most cases here is balance.
At this point you need to decide if you want to build the site your self, or have your webmaster create a web design for you. There are pro's and con's to both methods. As long as you are working with someone you know, and trust, your website should come out great.
So how do you know? How do you know if you have a great website, other then your friends and family telling you? Try to look at hard data. If you previously had a website, what was your traffic like? What was your conversion rate, bounce rate, etc.? If you are not familiar with these terms, look them up. If you did not have those kinds of statistics before, be sure to have them on your current site. I prefer Google Analytics, but there are a number of other options out there that provide a similar service. Once you have that data, compare it with the goals you set when you first started. How is your site doing? If it is meeting or exceeding your goals, then see what things specifically are making that happen. If not, then see what you can change. The internet, and web design will always be dynamic, and always changing. So be sure to update your site often. Check to see if it is still meeting your goals at least once a month.
Best of luck out there,