how much money a month does someone in the navy?

There is so much information in that answer it's almost impossible to actually answer. Short form: from a very little bit to a whole lot. There's a lot of variables too, like whether they receive an extra allowance to pay for rent and whatnot, etc. The website below lists all of what you're looking for, for the current year (until Dec. 31, 2010).

The lowest: E-1 pay with no BAH, BAS or separation pay or anything else. It's like $1,400 a month or so before taxes.

Highest: 4 star admiral (I hear they make something like $10,000 a month)

http://www.navycs.com/2010-military-pay-chart.html

Applies to all branches of services.