Be careful with ringtone apps... many of them have mandatory ads or require you give the app permission to have access to some items.
A better solution would be to go to a website that has FREE ringtones you can download. Installing them is very easy. On my phone, I first created a folder called Ringtones. Now, when I go to any website that has ringtones, I preview and download the ones I want... then I just simply copy them over to the Ringtones folder I created on my phone. When I want to change my ringtone to something I downloaded... I simply browse to that folder.. it's so simple and easy.
Here is a website with thousands of free ringtones:
http://www.zedge.net/ringtones/