The gravitational force between two objects is given by the following formula:
Where G is the gravitational constant, and r is the distance between the two masses.
Just google for G and you should be able to apply this to get what you want.
I am having a bit of trouble with this problem...
The gravitational pull of the Moon is partially responsible for the tides of the sea. The Moon pulls on you, too, so if you are on a diet it is better to weigh yourself when this heavenly body is directly overhead! If you have a mass of 85.0 kg, how much less do you weigh if you factor in the force exerted by the Moon when it is directly overhead (compared to when it is just rising or setting)? Use the values 7.35×1022 kg for the mass of the moon, and 3.76×108 m for its distance above the surface of the Earth.