How to change my ringtone on my iphone?

I have an iphone 4 and i want to make my own ringtone from my itunes library

We've done several ringtone-type videos recently. It seems that people everywhere are demanding easy and free ways to get ringtones on their phones. Domenic has discovered a simple way to create iPhone ringtones from your own music library by using iTunes.

Go into your iTunes preferences, and make sure you are using the AAC encoder. You can double-check this by going to Advanced, then Importing. Now, browse to the song you want to use to create your ringtone. Right click on the file, and choose Get Info, then Options. Under the Options tab, select the start time and stop time to create a 30-second clip. Once your 30-second clip is selected, right-click the song and choose Convert Selection to AAC.

Once the conversion is completed, you will get a 30-second song of the same name. At this point, you can edit the original file back to its normal length. Now, browse to your iTunes music folder. You should find the new clip in the same folder where the full-length song is. Notice that the clip has an extension of.m4a. Simply rename the extension to.m4r, then select it. It will automatically be added as a ringtone in your iTunes library.
http://www.cnet.com.au/how-to-make-your-own-iphone-ringtones-339291124.htm