How does a domain work?

I've been thinking about purchasing my own domain but then I realized I know nothing about it actually. Can somebody help me please? I would really, really love to be part of the .com community but I don't know what to consider in getting my own domain. Here are a few questions that I came up with:

1. Where to purchase and how much? I am gearing towards purchasing the cheapest possible.
2. How would it affect this blog on blogger.com? Will everything I've written be imported to the .com?
3. What's the guarantee that I'll get more traffic with a .com?
4. How do I design a .com? Any leads on good web designers that don't charge so much?
5. How long does a domain registration last? Is it a year?

All comments will be highly appreciated. Thank you!