Why wont it let you talk to people in halo 3?

Because you don't have a mic or you have the mic turned off. If you have an Xbox 360 there is a wire on your mic that connects to your controller. There should be a switch and if it's red or orange it's off. If you switch it the other way it's on.

---------- There is also the possibility that your family settings are not allowing you to speak over Xbox Live. If your account if registered as under 18, the adult will need to go into family settings and change communications to allow and such.