theres two ways to do this. the first is open system preferences, go to audio tab and input ...or output tab, either will work, and on the bottom there should be a dropdown list that says, use audio jack as....
My guess is it says output... just change the dropdown list to say input
I'm also assuming you are using garage band? if so go to preferences, and you can change the audi driver there, from system default.. to like line in. if you just do the garage band thing, the next time you use your jack it will revert to the the setting in system preferences (which is probably headphone).
if you use the first method you'd have to go back to system preferences and switch it again to use it as a headphone jack