There are many ways to buy an iPod nano. I will go through the ones I think are the best.
The most straight forward approach is to go to www.apple.com. Once you are at apple.com, click on the store option. Click on iPod nano right in the center of the page. The next screen is where you select what color and size you want. The page will then ask if you want free engraving. Click either continue (to skip engraving) or add engraving (to add engraving). You will be taken to a page that shows your cart. If the items in your cart are correct, click check out now. You will then go through the standard online check out and, if you successfully complete this screen, your iPod nano will be shipped directly to your house.
A second easy way to get an iPod nano is to go to your local Apple retailer. Here you can check out easily, without shipping charges. If you need help at the store, ask an Apple associate for assistance.
The final choice you have for buying an iPod nano is from a friend or neighbor. Make sure you check to see that they actually have an iPod nano to give to you. This way is not always as safe because they have used the iPod already and it might not work to the best of its ability. I would not recommend this choice unless you are very desperate.