On Modern Warfare 2, you earn weapons for multiplayer by achieving higher ranks in multiplayer.
Just kill people
Yes, there is online multiplayer on Call of Duty 4: Modern Warfare.
you have to get kills on your friends or on wifi the more kills the more guns
There are no golden weapons in Modern Warfare 2.
U go to multiplayer if no multiplayer u beat
Call of Duty World at War, Modern Warfare, Modern Warfare 2, Black Ops, and Modern Warfare 3 all have online multiplayer.
Yes You Can
The related link lists and described the weapons used in Call of Duty Modern Warfare 2
Unfortunately no, you can't play multiplayer on Call of Duty Modern Warfare 1 or on Call of Duty Modern Warfare 2 on the same screen. But on Call of Duty Modern Warfare 3 and Call of Duty Modern Warfare Black Ops you can play multiplayer on the same screen. There is an new Call of Duty Black Ops 2 coming out soon though, so I expect that to also have that feature, since its newer.
There are a total of 70 levels (ranks) in Modern Warfare 2 Multiplayer.
No you can't.
Sadly no.