iMessage help iPhone

Here is how iMessage works and what you can do http://support.apple.com/kb/HT3529

If you know the error you are receiving then we can help you more.

The more details you can provide the better.

Thanks

I did all is but I still can't send iMessages, I can receive tho.

The error is that "not delivered"

Are you on WiFi?  Are you logged in with your Apple ID? Is the person you are sending to using an iPhone? What is the error you are receiving when you try to send a message through iMessage?

I am on wifi, I am logged in with my apple id, the person have iPhone 4s with latest firmware !

He can sent me messages and I can receive them, but I can't send him back . Yes this is an error that I get when I try to sent a message from iMessage!