Hi Debra!
Setting up a blog on Yahoo! is really easy. If you visit your Webhosting Control Panel, go to the Create and Update tab. On here, you have the choice of using one of three platforms for your blog: Yahoo!, WordPress, and Movable Type. For beginners, I personally recommend using the Yahoo! blogging platform (because it's so easy to use).
When you set the blog up, make sure you DON'T set the blog as your homepage, or you'll find your store is missing
If you have any questions, shoot me a message or post here!