Lorelle has a great post up that goes over what you should be doing after you have created a WordPress.com blog. With two different ways to tackle starting your site, Lorelle goes over the typical approach and a longer, but more practical appoach.
If you choose the practical approach to starting your WordPress.com Blog, and you are ready to get serious about your WordPress.com blog, then here are the step-by-step instructions.
1. Make a Plan for Your Blog
2. Identify Your Blog
3. Set Up Your Categories
4. Write Pages About You and Your Blog
5. Choose a New Look for Your WordPress.com Blog
6. Start Blogging
7. Managing Your WordPress.com Blog
I really recommend reading this post, even if you are not using WordPress.com, as there are many great tips and tricks in there on getting yourself set up properly and ready to blog.