alliance chat channel

Sorry for a newb question.

How do I post in our alliance chat channel - I have plenty of horns but do not know how to use them.

you don't need horns for alliance what you do is click where it says world or private or whatever and then select alliance then you can talk

What santa is stalking about is, next to the field where you type the text. To the left of it is a button you can select which says World. Once you click it you have 4 options, from top to bottom:

Shield - Makes it so you can disable text being shown from Alliance, World or Private when being on the Global tab
Private - When you private message someone you need to be on this tab and start your text with /targetnick or /"target nick" depending on if their name has a space in it. Usually this is done automatic when you click on a persons name in the chat window.
Alliance - Select this to only send a message to your alliance.
World - This is the public chat, which costs 1 horn per message you send. Only option that costs horns!