There should be 2 lines marked. FULL HOT, and FULL COLD.
Fill to the full cold.. If you want you can leave the bottle in the truck
and check it after you have driven it.
In that case you would want it to be filled to the full HOT line.
If you have a suction pump/gun (available for 7$ at autozone)
you can remove all the old fluid from the reservoir and fill with fresh.
If you do that a couple times (seperated by some driving)
you can get mostly fresh fluid in the system which might make it quiet