SOP would be to drain out the coolant, you could save for reuse if it's okay. Then do a two part flush or take to a radiator shop where they could flush it with the high tech setup. Be sure to check the thermostat, drill a hole if you don't have one to vent air from under it when filling up. If you have just been using water, excessive mineral deposits, salts, etc.. could have built up. The water pump packing will probably go out after the flush, but that could be rebuilt or replaced.
I lost a head on a gmc 228 because I could not flush out the water passages good enough at home. After pulling it, I was really amazed at just how clogged the head and block were. Hope this helps.
