Lots of people I know have talked about wanting to start a blog, but not being sure where to begin. It's hard to get started when you feel like your blog has to be perfect from the start, that your first entries have to grab the reader because they're like your opening chapter. But the truth is, people get better at things over time. If you were starting a new instrument or sport for the first time, you wouldn't expect to perform perfectly on your first try. Keeping a blog is like anything else - practice makes you get better. The only way to get better is to start.
One thing to keep in mind is that you can always go back and edit previous posts. I make changes to old posts all the time. Since people can find your individual posts using search engines, any entry could be the first thing that someone reads. The other thing to keep in mind is that readers will see your most recent entries first. Don't worry about not knowing what to say at first - once you've figured out what to say and how to say it, your new entries will get better, and people will see those better entries first.
Finally, I had this blog for six months before I even realized that I could tag posts and check how many hits I've gotten. It was a long time before I added different pages and put my tags on display, and I just recently went back and added pictures to previous posts. When I didn't have many blog posts, I didn't see a need for anything other than the writing. Once I had enough blog posts that I didn't think new readers would scroll all the way back to my first post, I added the About Me page to replace my opening post. I added tags and the Best Entries page to link people to past posts that they probably wouldn't scroll to. The point is, you don't have to have everything figured out when you start. The best way to start a blog is to just start.