Wordpress is my favourite platform for blogging, it's free, easy to use and fairly SEO friendly "out of the box".
But there are some free plugins which you can install to make it SEO "perfect"
You can download plugins for free here. Once downloaded and unzipped you need to upload them via FTP to your hosting (or ask your designer/host to do it for you). They should be uploaded to the folder:
/wp-content/plugins/ <<<<<<< EDIT
The SEO Friendly Wordpress Plugins I use are:
Google XML Sitemaps - This will create a search engine sitemap, which helps Google to find your pages. The sitemap will automatically update each time you post. You can also submit this sitemap to Google Webmaster Tools. If you don't have and account you can open one for free HERE
Robots Meta - When you open your Webmaster Tools account they will give you verification code to paste into your blog. This plugin provides a field for you to add it without knowledge of coding.
For the advanced SEO'er it also does some other neat things like forcing a trailing "/" on the end of urls (which is a good thing, trust me) and facilitates adding "noindex" or "nofollow" tags to sculpt PR or reduce duplicate content issues
All in One SEO Pack - The most important plugin of all! This plugin allows you to add a unique title tag, meta description and KW tag to each post.
Redirection - An important aspect of SEO is to direct the www version of your domain to the non-www (or vice versa.) This is known as canonicalization (but don't fret about that). This plugin does it for you automatically!
For advanced users, it also has a brilliant 301 re-direct tool, so if you move or delete a page you can re-direct it to another page in seconds (and not lose any PR juice).
Google Analyticator - If you are running an adwords campaign you need to add tracking code - a doddle with this plugin