First, click the App Store. The App Store icon is blue with pencils in the shape of an "A". Once App Store launches, you will be at the "Featured" menu. This menu has some of the applications that Apple is recommending at the time.
Click "GET" under the application you want. For instance, to download the YouTube application, click the gray button under "App for YouTube..." that reads "GET". The button will turn green and read "INSTALL APP".
Once the application has been installed, you can open it from the same location by clicking the "OPEN" button. You will see an option to "Unlock Pro", which is a paid option. Like a lot of free applications, this app does allow in-product purchases.
To find out more information on in-product purchases before downloading the application, click the application name. It is a hyperlink. On the next screen, you'll see the "Top In-App Purchases" section at the right side of the window. In this case, it's "Go Pro" for $2.99 that you can click.