Oh, you meant the holding tank on your boat.
You have to either call a pumpout boat and have them come over to your marina or go to a dock with a pumpout station. They will have a hose that screws into the deck fitting on your boat—be aware that they have several adapters and only one will fit your boat's pumpout deck fitting properly. Then do the following:
1) Fill the head bowl with water.
2) Go on deck and remove the cap from the head pumpout deck fitting. Unlike the water cap, it won't be connected via a chain... so don't lose it.
3) Connect the hose to the deck fitting.
4) Flip the big handle on the hose.
That opens the valve at the front of the hose.... and vacuums the crap out of your holding tank.
You should see your head's bowl empty of water... this is the way you can tell it has emptied the holding tank completely, and acts to help rinse out the holding tank and hoses.
Once the water had been emptied out of the head's bowl—do the following:
1) Flip the big handle back
2) disconnect the hose from the deck fitting
3) Close the deck fitting opening.
I hope that helps...