Add New Host

Hi all,
I have created a cluster on a host. Now I would like to add a new host to this cluster but in the page “confirm hosts” there is a error
Registering with the server…
Registration with the server failed.
In the file / etc / hosts I have entered the data host that I want to add, while I start Ambari agent in that host. What are the other things I have to do or install on the host to add?
Can you help me please?