There are lots of things to consider when designing a web site, and here are some to make you realise it is not easy.
1) How would you design a web site so that it would look good on a 15" laptop screen as well as a 22" widescreen monitor (and everythng in between).
2) Are there any considerations when designing a web site so that it works in different browsers (IE, Firefox, Opera, Safari etc).
3) Are there any considerations when designing a website so it works on a PC, a Mac, a Linux computer, and other computers.
4) If you are going to put images on your computer is it best to use JPEG, GIF, BMP, PNG or something else ?
5) What considerations do you need to make for people with special needs (poor eyesight, lack of hand control etc).
6) Are you going to use pure HTML or use Cascading Style Sheets (CSS) as well?
7) What product are you going to use to design the site: Dreamweaver, Expression, Net Objects Fusion, or something else.
These are just a few of things you need to consider when designing a site, there are LOADS more.