The mother board is the main circuit board that all your cards and processors are attached to inside the computer case. If you take the side of the case off your computer then you should easily be able to discern the motherboard it is usually attached to the bottom or side of the inside of the case depending on what type of desktop box you have.
First thing I woudl suggest you do is get a can of compressed air and blow the dust out of the all the fans on your power supply, processor and any other fans you may have in the box, also blow out any dust from the processor heat sink, it is usually underneath the fan and is generally silver in colour it looks like the fins on a radiator.
This is what a motherboard looks like
This is what a heatsink looks like