Web design is a lot more complicated than most people realize. Sure anybody can design and build a web page these days but having a site that works as a whole is a much more difficult task. It takes a skilled designer to create a site that is coherent and looks like it belongs together and not like a collection of separate pages. Even most web designers struggle with this which is why you need to use somebody with a track record of success.
It should sound obvious that you have to make sure that the design of your website works as a whole, but doing it in practice can actually be quite a challenge. The main thing that you have to pay attention to is that you design your entire site with a purpose in mind. The site that you build should be built for a reason, you are after all trying to accomplish something with your site. It is important that you determine what that is and build your site accordingly.
The best way to make sure that your site that your site works as a whole is to sit down and lay the whole thing out before you start building it. This is another of those things that sounds obvious but that most people won't do. What usually ends up happening is that people build their site as they go, adding new pages whenever it interests them. The problem with this approach is that you will end up with a site that does not fit together. You want to have a coherent design for your site and this requires that planning to achieve.
There are a lot of practical details that you are going to have to address as part of making your site work as well. The most important of these would be to make sure that your site is easy to navigate. You have to make sure that people can find what they are looking for easily because you can be sure that if you don't your visitors are not going to stick around. A site that is easy to navigate should be laid out in a logical manner and have lots of links to help people to find what they are looking for.
One other consideration that you have to keep in mind when you are designing a website is that your site is your brand, you have to design it that way. This means that all of the pages should have a similar look. They don't have to all look the same but they do have to be similar enough in appearance that people will be in no doubt that they are still on the same site. This will help to establish your site as being yours and help you to stand out in the minds of your visitors.