Replying to a message is very easy.  Click on the message to which you want to reply.  In the case below it has been decided to reply to the message from Les Stephenson.

To reply, click on the Reply icon. To reply to all the people that received the message, click on Reply All.

The message box opens with his address already there, although it may be 'underneath'.  Write your message in the area indicated.  It is wise to delete any part of his message which is not required in the reply.  When you have finished the reply, just click Send and it will go back to him. 

A note on e-mail netiquette.  Do not correct grammar or spelling mistakes in other people's messages.  In this case Les made a mistake in the subject - it should read Write your subject here.  It is very easy to misinterpret e-mail messages, so be tactful and careful.  Do not tell him of his mistake.