I can across this card while deck building, I have never used cards that cost 'X' so I was just wondering how would I pay for this card?

The value of "X" is variable, and the meaning of the variable will always be given on the card text. For this one, X is the cost of the ally that you put into play. So if you put into play a Northern Tracker, X is 4. If Gleowine, X is 2.

Another example of a card with cost X is Gandalf's Search. In that case, X is the number of cards that you want to search through.