I had a similar problem on a car I bought a month ago, you say the the coolant tank is empty so fill it to about half an inch above the max level, check that the cap is screwed down these caps can be a source of a minor leak, check the oil level using the dipstick, there is a bleed point on the thermostat housing on later models and if the original has ben replaced on many of the older cars. It's easier to access than the heater bleed valve.
If the oil level is at the max mark and clean then you probably haven't got a problem with the head gasket but if the stick shows signs of Mayo then it can be head gasket or water/oil cooler, the latter is an easier job to change. For now ensure that levels are correct, warm it up and keep an eye on things, how long was it between all being ok and noticing the coolant level is so low?
If levels are ok and stay that way with no sign of fluids mixing after running to temp, check daily to see if there is a change, you could have an airlock in the heater system they're not easy to bleed properly.
To find the bleed screw in the thermostat housing, looking into the engine bay from the front go to your right and between all of the pipes for the intake system and the cylinder head towards the front look down and you may find a plastic screw head, you can lose it a few turns and watch for water seeping out when you see water coming out tighten it but be aware that the screw and thermostat housing are plastic so don't be heavy with the screwdriver and don't remove the screw it's difficult to put it back with everything in place. Hopefully it's just an airlock and you're not losing coolant.