Child support - CS is calculated based on the income of both parents, usually as shown on their most recent tax return and/or last two pay-stubs. If you have not been working because you have been a stay at home mom, you will be expected to find employment again and both parents will split the cost of day care.
To get CS you need file a request with the family court that serves your area.

SS - sorry I am not familiar enough to give a good reply.

Keeping son away from father - NOT going to happen. Unless you have documentation from the authorities of actual abuse to the child the Court will find it is in your son's best interest to have at least SOME visitation with his father.

Keeping son away from in-laws - while son is in father's custody he has the right to visit with his family. However, the in-laws do NOT have the right to visit with son when he is in your custody...unless they file a request with the Court and get an order for visitation. If their request is granted you will have to follow the Court's order.