When you first start blogging, you'll find a nice assortment of templates to choose from, to serve as the foundation for your new blog. After you've developed a few blogs, or tweaked one blog for a while to get it just right, you may decide that the Blogger provided assortment isn't so comprehensive as it looked earlier.
What to do now?
One solution would be to go wandering through the blogosphere, looking for other bloggers who have experienced the same frustration as you, and see what they did. In some cases, they'll offer the results of their efforts, frequently for no cost to you.
That's a third party solution - and third party solutions have both advantages and disadvantages.
Maybe you decide that you know what you want better than they do, so if they could create their own, why shouldn't you? And now, the fun begins.
What do I do now?
If you're completely new to this, then learn how templates work, and why they don't. Take an existing template, decide what you don't like, and figure out how to change what you don't like. None of this will be wasted effort, because all of what you do will be experience when you build a template from scratch.
After you've been tweaking and removing what you don't like from the existing template, start putting what you do like into a clean template. You can develop and test your clean template in a second blog, while you work on your existing template in your first blog.
When maintaining two blogs becomes too much work, and you can get more done by making changes straight into the second blog, there's the start to your first template developed from scratch. Keep the first blog, as a baseline, and start working on the second blog.
You work on the second blog in reverse order to how you previously worked on the first. Use the pieces of the first as models, and add the pieces into the second. Where you like a feature in the first blog, simply copy the code into the second. Where you want something different, write the code from scratch. By the time you've gotten this far, you already know how templates work, so now you learn why they don't work as you like them, and how to make the pieces that you want.
Again, just one step at a time. Backup your changes, and test your changes, with anal regularity.
Oh yeah, for testing your changes, there's the New Blogger June/July 2008 feature that lets you export comments / posts from one blog, and import them into another. So work on one blog, keep the second blog as a baseline, and copy the blog contents back and forth to create a nice uniform test environment.
And enjoy the learning. Blogging can be fun, if you relax.