First you will need software with which to build it.
If you want to use a WYSIWYG (the easiest way to do it) there are several to choose from: you may already have Frontpage; NetObjects Fusion was given away on the PCA cover CD/DVD back with the April issue; Nvu is Open Source (free) click here .
If you want to learn HTML/CSS and do it from scratch W3Schools is a good place to start click here it gives you tutorials and interactive examples (you rewrite the code and see what happens). This is quite useful even if you do go with the WYSIWYG approach because you may need to tweak the code that the software produces.
If you decide to start from scratch using HTML and CSS you will need an editor; HTML-Kit from Chami is free for personal use and has loads of add-ons for all sorts of other languages so it will grow with you click here