iMassage is inactive

Hi,

I am new to the MAC OS and trying to get the iMessage working but without any chance, once I try to send an iMessage to one of my contact, i get a request to login since the iMessage is inactive, I logged in successfully but still I am not able to send any messages as the problem is still there.

Will you please be so kind and let me know what to do in this case?

I am using MacBook Pro 13" early 2015 with Yosmite Version 10.10.5.

Looking to hear from you