Well, I am still new to this, but I have been wanting a category section for blogger.com for a long time. Finally, today, just in time, I googled "create categories for Blogger.com" and learned that del.icio.us now makes it possible for users of blogger.com to have categories - and it's so much easier than the other previous suggestions.
Here's what I did, maybe it will work for you too. I already had a del.icio.us account, but if you do not, just sign up for an account at http://del.icio.us . It's easy. Remember to add the "my del.icio.us and "post to del.icio.us" links to your favorites or link section (near your address bar).
First, you might want to decide what the names of your categories should be.
Now, simply go to your blog, click on your "post to del.icio.us" link and add your blog to your del.icio.us account, add all your category names as "tags".
Go to your del.icio.us account, click "help", click "Tag Rolls", now simply choose how you want your categories displayed in your blog and copy and paste the code in your template. You're done!
I haven't figured out a better way to do this yet, but I simply click on the page of my post, add it to my del.icio.us account with the proper "tag" and it is automatically categorized.
Del.icio.us, evidently, also pings your post for you because I went to use ping-o-matic and my post had already been pinged.
I'm still experimenting with it, but it's a great addition to my blog.
So that's it. Now blogger.com bloggers can blog with the best of 'em!