Question: how to set a song as a ringtone on iphone without garageband?
Answer: Find the song you want in iTunes.
Right click on it and select "Get Info" (Apple+I also works). ... 
Adjust the "Start Time" and "Stop Time" values to shorten the song down to the part you want to use for the ringtone (45 seconds max). ... 
Play the track and make sure you're happy with where you've set the start and stop times.