Why do you want or need a website? You need to know what it is you are selling and who is going to buy it (demographic) - this will help determine the content, detail, style and presentation of the site.
What do you want site visitors to do? You need to be clear as to what you want from your site visitors - buy something, opt-in to email or newsletter, gather information or pricing? This dictates the functionality of the site.
You need to have a good idea of what you like and dislike about other websites you have seen. If you can come to a web designer with at least 3 examples each for likes and dislikes this will speed up the design process considerably.
Be honest with yourself and your designer up front so expectations on both sides are clear. A £99 website won't look or work like a £999 website! If you buy a square and cut too many corners it always ends up being a circle....