I heard the price of a postage stamp is going to change this year. Does anyone know when?

Not a radio tower or anything but an actual building.

I need to know the top 5 countries with the most cell phone users in the world. Can someone help me?

I need a list of the top 20 most populated cities in the United States.

I want to give my brother some extra money to mail some letters in the US to family for me. How much is it to mail an envelope? 40 cents?