Assuming there are no adjustments on the tuner, then you will need an attenuator pad. Check out this webpage for some info - http://www.uneeda-audio.com/pads/
If what you see there sounds like too much trouble, then hook up your tuner to an amplifier and connect the speaker outputs to you computer sound board. Start off with the amplifier volume control at the lowest setting. You will find that you will need to run the volume contol at almost a quarter of the way - this is because there is an impedance mis-match. The amplifier speaker output is at an impedance of about 8 ohms; while the computer sound board line-in jacks are built for about 1000 ohms. This mismatch will not effect the recording quality, it only effects the amount of power that will be transfered to the sound board. The attenuator pad will make for a lot less space used on your desktop, but if you have an amp handy - that will work for you too. You might also be able to buy an attenuator pad at Radio Shack.