1991 Chevy Van What do you repair for this?

Initially, we lost our heater, & I am not sure if it's related to needing to fill the radiator more often or not. It then started with the check engine light coming on every so often but no problem we could discern. Then we started leaking a lot of water & by the time we got home, water would just pour out of it. My son knows very little about cars & spouse & I are too disabled to look, but he said that there is a tube (he's not sure if it's metal or tubing) that connects to the engine & that's where the leak is. In looking up how the system works, it seems to me that it could be either the thermostat or the pump, or maybe just a bad hose, so although I learned a lot from your site on how a coolant system works overall, I still don't know what's broken. I'd really like to know what I'm looking at for repairs. Can you help me understand what's going on? Thanks.