In 8 First Step SEO Tips for Bloggers Darren answers this question:
What are the first steps to optimizing my blog for searches?
Darren expands on his eight tips, in short they are:
1. Content is King
2. Anticipate What People Will be Searching For
3. Titles Titles Titles
4. Keywords in other parts of your post
5. Link to Your Own Posts
6. Links from Outside Your Blog
8. Readers Begat Readers
For help with item 2, what people search for, I use a plugin called WordPress.com Stats. It gives me the statistics I need and is easier on your blog than Google Analytics.
Under item 7 (plugins) Darren links to an older post, 9 SEO Plugins Every WordPress Blog Should Have, but the plugins I checked were not updated for WordPress 2.8.
Update January 2017
This post is old (2009). Here’s another post that is useful:
Alternatives to 66 of Your Favorite Dead WordPress Plugins