Once you have iTunes installed on the computer, at top under the apple logo there is a button called "iTunes Store", click that the store will then load in the centre window of iTunes.
From here you can browse by category and see trends etc, if you type in the artist or song name your seeking in the top right search bar and then press enter a list of songs matching your criteria will load on the iTunes store.
You can click the song to listen to a preview before buying it, and once you chose the song you wish to buy, click on the price at which point it will take you through the process for registering a card on your account to make payment.
If your referring to downloading music on to your MP4 device from iTunes this isnt supported in the current version of iTunes. In this instance you use the sync software which came with the MP4 device and point to the same directory as iTunes. There are other products you can use such as iTunes agent which can be found here, a video explaining how it works can also be found here.
Please let me know if you need anything clarified.
No problem so you want upload music on to the MP4 player, there is a video here which may be helpful to watch first.
You upload musci as follows