Some great advice being given here.
My advice would be to write what you know about, if you've got a hobby that you want to share with other people then you are halfway there. Don't bother choosing a subject that you know nothing about, it will be boring and probably not particularly interesting to people who know more than you.
In my opinion, the most successful websites are ones that fall into a "niche" if you haven't got thousands of other people to compete with them. It's a lot easier to get your website into a position where people will come across it on a regular basis when doing searches on the Internet.
I've seen too many people using ridiculous headings for their website, trying to cram in as many keywords is not the right way to do it. If you've got a website that contains lots of useful articles then choose two or three major keywords and make the heading of the page and meaningful, almost like a page heading in a book. Doing this will attract people to your website because they will see something that makes sense, rather than a bunch of keywords that doesn't actually tell you what's on the page.
Just my two cents. :-)