Can I be charged with an MIP if I'm at a party, but I'm not drinking?

No.  You cannot be charged with an MIP solely because you were at a place where other people possessed or consumed alcohol.  However, if you have an alcoholic beverage in your hand, even if you haven't taken a sip, you can be charged with an MIP. 

If a police officer asks me if I have been drinking do I have to answer?

Will I get in trouble if I'm under 21 and I call 911?