From the moment you insert the USB cable into the phone while it's attached to PC, the USB on the phone will auto-activate and the phone will be visible in My Computer as a Mass Storage device. If you don't have its USB drivers, I recommend you visit the official support website to download them. Install the drivers, then plug the phone and USB will be turned on automatically. As you have Android 2.x, you will also need to press the Connect USB Storage button when it appears.
