Do I have to answer the Plantiff's Interrogatories to Defendant? It's asking questions such as the last four of my SS number,grosss monthly salary & other stuff.I have already responded to the summons & have a court date.

Probably. It depends on what the case is about but, assuming your finances are relevant to the case, the other side is entitled to conduct discovery so as to find out that information in preparation for the trial. You are also, of course.

Yes, but you can do the same to them.