A great way to animate prims in Second Life is to use Puppeteer. This tutorial takes you through the steps of animating a bear. Although this one isn't too good....I will clean it up and add a better one later.
**NOTE** Puppeteer is no longer available for sale in Second Life. You can find scripts here and here that will do the same thing. (The second link will take you to thescript created by the Puppeteer creator, Todd Borst.)