Just got an iphone and I'm trying to download an app?

So just got a brand new iphone (love it!) but I'm trying to download a ringtone maker and it says that I'm not authorized on this computer to do that? It says I need to authorize it, and I have no idea how to.
Thanks

Added (1). Here's the thing, I can't find anything that says Authorize Computer anywhere.

Added (2). Ahh I'm such an idiot, it's right there.

Open iTunes > Store (top left)> Authorize this Computer > Done. You'll just have to put in your password and you're all set.

Also, you don't have to download from your computer. Just go into the App Store app on your iPhone, all you'll need to do it click on buy & install.

haha you're welcome. Glad I could help