Where is my truck leaking water?

It’s a steady leak and when we put the cap back on and start it there’s a little white smoke coming out the exhaust

More than likely you have a heater core leak along with a possible heater hose leak as stated above.

Also, there could be a leaky head gasket with reference to the white smoke. Check your dipstick for discoloration and possible water in the oil pan. This would be the worse thing to happen, the heater hoses and possibly heater core are not really a big deal.

Thank you for your help

Would the heater core still leak if the truck isn’t running? It’s leaking coolant when its just sitting and isn’t started but still leaks when it is started