Multiple files in different namespace

Question

Hi All, I am just the beginner of C#, and I try to write very simple code of C#. MY question is: if we have more than one file, and each file has its own "namespace" so, how does the main file calls or uses all of there namespace. Ok, let's say this in the simpliest way. Example that I have one file( let's say A) with its namespace named H which contains a class Hello. And, I have another file named Main. which have the "void Main". Now, how do I use the class Hello of the file A Plzzz help. Thanks in advance Tuine