Greetings, for apps use one of the amplitube apps or GarageBand. Get an apple camera connection kit and use the one with the USB connector. Get a chord like the one used in RockSmith (video game). Plug it all together and you can pipe it out to headphones or through the headphone jack to your Marshall amp.
iRig also makes a plug in device that goes in the headphone jack. You can just use the regular guitar cord. I've seen them as low as $12 including shipping.
Rock On!