Ensure that the power to the Wii U console is off, then insert the USB plug of the device into one of the USB ports on the front or rear of the console. If you are using a self-powered HDD, ensure the device's power supply is plugged in and receiving power.
You will be prompted to format your USB device when the Wii U console recognises the USB device after start-up. You can also format your device by going to System Settings, then Data Management on the Wii U Menu.
Please note that after formatting a USB storage device for your Wii U, it is formatted exclusively for this Wii U, and cannot be shared as USB storage media with other devices such as personal computers or another Wii U console.