I've been making ringtones with a program I got a long time ago but it's been a hassle. So I tried Audacity after I saw it posted around here and decided it'd be nice to make a tutorial for everyone.
Download Audacity here:
http://audacity.sourceforge.net/download/windows (Audacity 1.2.6 installer)
Download LAME MP3 encoder here:
http://lame.buanzo.com.ar/ (v3.98.2 exe)
Install both programs then load Audacity.
1. Import the song: Project > Import Audio.

2. Play the song, then select which part of the song you want to make as the ringtone: Select a Start then drag it out to an End.

3. Export the song: File > Export Selection as Mp3..

NOTE: For first time users, it will ask to locate the LAME mp3 encoder. Go to the location that you installed it (C:\Program Files\Lame for Audacity) and double-click on the lame_enc.dll
4. That's it!

Sending/sharing options:
1. You can transfer the song by PC to Phone, drag-n-drop the file to My Ringtones.
2. Email it to your phone:
1AREACODENUMBER@mymetropcs.com (
15555555555@mymetropcs.com)
3. Bluetooth it over to another phone.