Go to your dashboard. Click on "Presentation". Click on "Theme Editor". Click the template you want to change. Should be labled "Main Index Template" or"Index Template". In order to modify any of thee files within WordPress you must CHMOD the files to 666.
If you're going to deploy a lot of blogs I highly recommend you pick up a copy of the Semiologic Pro WordPress CMS (
http://www.semiologic.com/solutions/sem-theme-pro/). It's what most of the internet marketing community uses nowadays for setting up mass SEO'd blogs with a slew of plugins to make deployments a snap.
My Free Advertising Blog in my signature uses this CMS.