For the metering in manual mode, fill the frame with what you want to be properly exposed.
Press the shutter half way down and then look at the little meter and see where the arrow is pointing.
If its to the left, you need more light - done by a wider aperture, longer shutter or higher ISO ... or a mix of the three.
If its to to the right, you need less light - done by the reverse of above.
With the above, if you meter for the bright side of the moon and your arrow is to the left, then you need more light.
What the Aperture and Shutter priority do is that you set 1 of the two values and the camera will set the other one to get that arrow in the middle. While this is great for many occasions, the numbers may not give you the effect you want, hence the need for manual mode. Both of these functions however allow you to under or over expose your picture...