Windows 7?
1. Connect your phone to your computer with the USB cable that came with the phone.
2. On your phone, a prompt should appear on the phone asking you to select either "Charge only" or "Mass storage" modes. Select "Mass storage" and tap the "Done" button at the bottom of the screen. A prompt will appear titled "Turn on USB storage." Tap the "OK" button.
3. At this point, you may see an Autoplay window pop up on your computer. If so, click on the option which says "Open folder to view files." If you aren't prompted with an Autoplay window, you should be able to open Windows Explorer's "Computer" window. You should see two new drives listed under "Devices with Removable Storage" which were not there before connecting your device. Double-click on the first of the two newly listed drives to open it.
4. At this point, you should see a listing of folders on your device's internal "sdcard" beginning with ".android_secure." Create a new folder here and name it "music" or whatever you choose. Copy your music files to this folder. Disconnect your phone.
5. On your phone, your files will be saved to /sdcard/music or /mnt/sdcard/music (unless your named the folder something other than music) .
6. Open the "Music Player" app which came with your phone. You should see your music files available here.
One recommendation: If you have a removable microSD card available to you, then save your music on that instead so you don't use up your phone's internal memory. (You would need to remove the back of your phone [where the battery is] to access the microSD slot and insert a microSD card.)
I hope this helps.