1. Momentum Canceling is a survival technique that kinda shortens your flight distance, enabling you to live longer. After you're hit, you exectue your fastest aerial, in Ike's case, his Bair, then fastfall, and double jump.
2. DI is short for Directional Influence, which also helps you live longer, and also get out of multihit moves. After a hit, the direction you tilt the control stick will alter the direction you fly in. Every attack is different, so I can't even begin to get into detail.
3. In most cases, after a JC, you're gonna start another jab combo. Also, a grab is another good option. Other less common choices are Up-B, Forward-B, Dtilt, Ftilt, Dsmash. it really depends on what your opponent does.
Was that clear enough? If you have any further questions, I'd be glad to answer them.