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Just plug your phone into your computer using the USB cable. If a notice about connecting as storage pops up, then select it. If it doesn't, you should see a USB icon in the notification bar. Pull down the notification bar and select USB Connected and then connect as storage.
The Phone will show up on your PC like a removable drive. Create a folder called music and copy all your mp3's there (just for organization -- it doesn't really matter as Android will scan everything for media.)