You can turn your Mp3s into ringtones for iPhone/iPod Touch 4G without any software other than iTunes and something to clip the song with (I think you can also do that in iTunes if you know how).
You need to cut the song to the part you want as a ringtone (no more than 30 sec). Then you simply import it into iTunes, convert it to Apple Lossless (AAC), find the AAC file and change the extension from m4a to m4r. If you're on PC... you have to go into folder options and unclick "Hide File Extensions"