If you don't know it already, you should read up on html and css. Some php may help also.
If you prefer, an easy way to start is find a free CSS template and work upon that.. once you get the hang of it you can make your own from scratch.
There are tutorials on skinning phpvms here: http://www.phpvms.net/tutorials
The way I usually do it is make an entire example home page then:
• Decide where the content begins, copy everything above into header.tpl.
• Decide where the content ends, copy everything below into footer.tpl
• Copy everything left into frontpage_mail.tpl and add the code for the content (news, new pilots etc) where your example content is.
These go in lib/skins/yourtheme/
et voila, you have yourself a skin. Just need to edit other template files (found at core/templates/) if necessary and place them in your theme directory.