You need to pick a tone/mp3 you have on your computer and convert it to a .wav file with any of the free programs you can find via searching on google or softpedia.com.   Once you have it as a wav file you need to move it to your phone (plug in your phone to the computer via usb cable, open my computer and click on the "Mobile Device"....then click the windows folder which should be listed after you click on the Mobile device) and place the file directly into the windows folder.  After you've moved the file to your windows folder on the phone...unplug the phone and go to where you chose you ringer and you'll see the new tone listed.   
There might be easier ways but this is the most direct way I found.  

There's a much easier way if you have the HTC media player:
1) Copy the MP3 or WAV file to the storage card or device memory via the ActiveSync method above.
2) Open the song/file in the HTC Audio Manager program
3) Use the right hard key for options and choose Set As Ringtone
4) You can choose to make it your ringtone as is, or trim it
-OPTIONAL-
5) Use the left hard key to select Trim
6) Select what part of the song you want to use by dragging the sliders or using the arrows to set the seconds.
7) Press the left hard key for OK