If you can design, but you cannot code, I would recommend hiring a skilled coder slice your layout and produce your theme (quick solution). Or you could spend a few days/weeks on
http://www.w3schools.com and teach yourself the basics of HTML and CSS. As far as the backend goes, there are tons of great CMSs you can use as the core of your site, so why re-invent the wheel.
http://opensourcecms.com
Now, about subject matter.... That depends. What do you know the most about? IMHO, I would pick a topic you know the most about, or maybe even you want to learn the most and I would do one of the following:
* A community site with a forum (usually best if you are very knowlegeable in the topics covered or have a lot of friends who are).
* An information portal where you publish articles and resources. These are a little tougher to drive traffic to, but are a nice addition to a community site.
HTH.
James
