Updated: Jun 9
You know what content you want on the website but have no clue how to present it to the user.
You know what content you want on the website, and you have the layout in your mind, but you don't know how to implement it.
In both cases you will need to explain your thoughts to the web designer. Although most people who read those lines are probably thinking that being in the second situation is better then being in the first situation. However, real life experience shows the opposite to be true. Giving a web designer the complete freedom of action regarding the web design based solely on the website content is usually a smart thing to do.
If you know of a website that has all the features you want or need and/or a site that looks the way you want your site to look, be sure to give the site url to the web designer. Doing so will give them some idea of what you want. You will both be looking at the same thing but will actually look at it from a different angle. Therefore, it may be better to give your designer more than one website as an example. The more websites you find that can express your feelings and/or needs, the easier it will be for web designers to understand your intention without you having to use a single term. Chances are that you won't find a single website that has all of the feature you want. After all, if such a website already exists there would be no place for your new web site to be born. Use several websites to express the different features you want. Spend as much time as necessary until you find just the right websites to provide examples of your needs. Doing research at this stage will definitely save you a lot of time later trying to point the web designer in the right direction. Although you are the one who needs to express yourself to the web designer, you must learn to listen to them as well. When they use technical terms, ask for their meaning. Do not finish any part of the conversation unless you are absolutely sure that both sides are on the same page. Remember that when a web designer speaks about the temperature of a color, he is not talking about the next day forecast.
Remember, you hired a professional web designer because you want a professional looking website and you couldn't do it yourself. So, trust the web designer's judgment when they tell you something you want won't work or isn't the best way to accomplish your goals. After all, you are paying them for their expertise. Don't try to tell them how to do their job.
It is OK to require that a web designer gets your approval each step of the way so you can tell them if one of your goals isn't being met. Also, if you really don't like how something looks and want it changed, tell them immediately. Don't wait until everything is done and then decide you don't like it.
A final word about cost
You have agreed on what needs to be done and the web designer has given you a price quote. Simple modifications and bug fixes are usually included in the price. However, other major changes or outright revisions may or may not be included. Make sure the agreement states what is included, what constitutes a revision rather than a fix, and how many changes you can make after delivery without incurring additional costs.