If you've never worked with the Web, you may be in for a bit of a rude awakening.
By its nature, websites are not going to look the same on every computer.
You are not going to be able to control exactly how they look.
Colors especially are going to vary wildly.
Your choice of fonts is going to be limited.
Now that is not quite 100% true. The problem is, the cure is usually worse than the disease.
You can have multiple versions of your website layout, and detect what screen resolution the website is being viewed at. It is more expensive, and it doesn't address issues of different screen size.
You can put all your text in images so you can use whatever font you want, but that is expensive and the search engines won't be able to read the text.
You can build the entire site in Flash, so everything is exactly placed and looks just the way you want it to, but then the search engines will barely know your site exists (if you are IBM or the website for "Shrek 3", who cares. But that is not most of us).
You can view your website on different browsers - different versions of Internet Explorer, Safari, Firefox, etc. - and carefully isolate every instance where one browser displays things differently than another. Then tear your hair out developing ways to force that browser to display it the same as the others, and the way you want it.
You get the idea. The web is much more a matter of artistic compromise than print media where you can pick your paper, specify color exactly, use print proofs to make sure it is exactly as it should be.
Deal with it.