Tricked Out

I have a samsung t609.. I wanted to put mp3's that i transferred on there as my ringtone... when i transfer it it can only go to the memory card then i select set as ringtone and it moves it to the phone but it says file is too long to have as a ringtone.. I was able to get it to work last week but i forgot what I did because I was just playing around til it worked.. If you know what to do, please help!!!! thanks