The App Store is Apple's application store, built in to iTunes, that allows you to download and install new apps for your iPhone or iPod. There are thousands of applications to search through and choose from, allowing you to customize your iPhone or iPod Touch usage as much as possible. You access the App Store through iTunes. While there is a general App Store for Macs distributing Mac apps, it's not available for Windows users.
Open iTunes from the "Applications" folder. Download the application if you don't all ready have it installed.
Click "iTunes Store" on the left.
Click "App Store" along the top.
Click in the "Search Store" field and enter a search term, or alternatively browse through the applications until you find something you like.
Click "Free App" or "Purchase" when you find an app you want. Enter your Apple ID information or sign up for an Apple ID if you don't already have one. Enter purchase information if you are purchasing the app. Next time you sync your device, it will install the application.