My car has a problem :(

I have a Toyota Lite Ace. Since yesterday my car is not moving like the way it should be. Its speed degrades that even the slowest car can overtake me. :( Before it happens, I've been hearing some crazy noise under my car that i don't usually hear. Too bad for me because I'm only just a driver. I don't have any idea on how to diagnose some problems on my car. The main reason why it happens because I missed to check the temperature gauge. I only notice that the temperature is extremely hot when I at home and then I randomly checked the gauge if its hot. When i saw the gauge, I checked the radiator. I felt that it was hot. I put some water to cool it off, but the water will just dried up instantly because it was too hot. So what I did is just let it stay overnight to cool-off. The following day, I put water on it. The tank is empty and I just put 4 liters of water. After that I drove from the house to the office. Then I realized that the speed is moving slow. When I'm in the 3rd or 4th gear, it's just like I'm in the 1st gear. When I'm traversing an uphill, my car is moving slow when I'm in 3rd and 4th gear where usually it's just ok before the overheat. I'm planning to let my car be checked by a mechanic tommorow. I just hope that if there's a need to repair, it will not be costly.