If there is a current child support order, then support is
due to the date of the Order if the parent is not making the court ordered
payments. You can file a Complaint for Contempt requesting that the
Judge make a finding and issue sanctions for failure to pay the obligation that
was either a court Order or Judgment and you need to prove that the Order was
clear and unambiguous.  Secondly, that
the other party unknowingly violated the Order. 
Third, you have to in order to prove to get sanctions have to show that
the defendant had the ability to pay. If you can prove that support has not
been paid as required by the Order, you will be owed money retroactively to the
date of the court Order and there can also be interest and sanctions applied.

If on the other hand you are filing to have an increase or
decrease in the Child Support Order you have to file a Complaint for
where you request that the court, make changes to the prior Order.  You have to prove that there has been a
significant material change in circumstances and that the change requires a
change in the Order.  This would occur not
only after you file the Complaint for the Modification, but when you serve the
Complaint.  There is a strong incentive
to get your Modification filed as soon as possible and have the other party
served as soon as possible because retroactive child support cannot go back
earlier than that.