connecting my guitar to my mac

hi, i have an electric guitar and this is what i want to do. Connect my guitar to my mac and through garageband add effects and the sound to come out of my amplifier. my problem is: i have connected my guitar to the imac line in with an adapter and my amplifier to my line out (headphone input) with an adapter also. i can now here my music or videos my mac plays through the amplifier and i can RECORD my guitar in garageband and AFTERWARDS here it through my amplifier but NOT WHILE PLAYING. any help? thanku

You need to turn on input monitoring for the track, which can be done near the bottom right hand corner of the screen IIRC.

Here's a thread from just a few days ago.

Searching the forums before you post can really help you find your answer quicker than waiting for someone to reply to your thread.