I emailed an attached mp3 file from my Comcast desktop account to my gmail and recieved it on my droid but all I could do is press a preview button and listen to it. I could not figure out how to save the file to the phone.
USB.
Connect the USB cable that is the cord of your charger into a USB port on your computer.
When you connect the phone, you get a notification about USB mode.
Select mount.
Confirm mount.
If autoplay comes up, select open folder to view files.
If not, go to my computer. Your phone will be listed as a device with removable storage.
I'm not sure if there's already a music folder or not. If so, I believe the music player will find your mp3 wherever you put it on the SD card.
Open the folder in your computer where the mp3 is and right-click it and drag it to the folder you opened to view the phone. I only suggest right click so you can be sure that COPY is bold in the popup menu that will come up instead of move. If copy is bold, you can simply drag and drop files by left-clicking. By default, it should be copy. Just don't want you to accidentally move it and not have it on the computer anymore.
When finished copying any files you want...
On your phone, pull down the notification tray and select he message about USB/mass storage.
Select to unmount.
If you want that mp3 as a ringtone, the easiest way I've found is to open the music app, locate the song name, long-press on it and select set ringtone.
This will permanently make it available in your stock ringtones Chooser and then you can set it for individual contacts if you wish.
Hope that helps! Any questions, just ask!