hi.. Im a beginner in electronics. Im always tensed while i try to connect arduino board with other circuits, as i think whether the impedance matching issue may cause any damage to the controller. Please tell me a better and safer way to interface with any kind of loads, also the effective method to measure impedances of circuits.

Thanks in advance....

Welcome! I'm not sure if impedance matters at all as much as DC resistance. Tell us what you want to hook up to your arduino we'll tell if it works or it smokes your arduino.

There is no measurement substitute for knowing what you are doing. You should be able to calculate the load of any device from its data sheet using ohms law.