Locate top radiator hose, follow it to the engine, undo the hose. What it is connected to is the thermostat housing. Undo 2-3 bolts connecting it to the engine block. Lift out the thermostat which is a diamond shaped unit.
Replace it with a new one. And refit the pipe. You might need to bleed the radiator in case of any air bubbles. Run the car with the radiator cap off. Top up the coolant level until the fan kicks in twice (20 mins).
Your relays are or should be in the engine compartment under a plastic box or under the dash board on the driver's side or on the side of the dash board on drivers side. I use to own a 2003 BMW 325i sweet cars
You need to find out if your car is leaking water onto the ground or if it is consuming it internally.
Here is a suggestion: When your car engine is cool, top off the radiator. The next day, before you start it, make sure the radiator is still full, and check under the car for puddles to see if it has leaked. Next,find a dry patch of pavement and start the car and let it warm up. Observe if anything leaks to the ground . If so, you can try to safely see the general location of the leak,to help figure out the problem. Look over all hoses, the radiator, and the water pump area for signs of wetness. If nothing is noted, re check the radiator level after the car cools again. If it is consuming water internally, then you most likely have a head gasket problem.
A ignition is in two parts, where you put the key to turn it and a second part that is the ignition itself. Between the two parts there is a small flat pin or bar. This is where your problem is, the pin/bar is broken. You will have to replace the ignition to solve the problem.