1998 Ford Ranger Radiator overflow, green sludge, oil in ra

Owned the truck for a year. Constant loss of radiator fluid. Checks show no leaks. Today radiator overheated after driving for 15 minutes with smoke coming off the engine. Let sit for hours while at work. Radiator fluid is now a green sludge. Re-filled radiator with water to get home. Oil low, but was just changed 2 weeks ago. Evidence of oil in radiator also. Truck starts up and sounds fine but after driving for a about 10 minutes overheats again and loses power. There is also a clicking noise. Ended up towing home. It seems like we may have started with a radiator problem that may have led to a cracked head gasket? But, I really know nothing about cars. Where do we start to figure out all the problems?