Each month that goes by generates its own judgment for unpaid past support when you have an order requiring a parent to pay support. If there is no order, you need one. If you cannot afford a lawyer, go to the Family Support Division of the prosecutor's office and get one. They don't charge you for this, because of the savings to the State.
If your ex does not work, is he receiving social security? The child support order can require payments from his benefits.
So in answer to your question, the first thing you need is a order for child support. If you have that all ready, then your options are to ask the State (DCS) to go after the support for you, or hire counsel to get the support for you. DCS cannot go after interest, but private counsel can.
The King County Bar has a program to help family law litigants. You can get more information here: http://www.kcba.org
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Great answer above. Also, you should sign up for support enforcement to do payment and collections. They will seek to suspend his driver's and other state licenses if he does not pay for 6 months. They also capture his tax refund.
Also, file motion for contempt and get a judgment for back support. Once you get a judgment, you can execute on the judgment or hire an attorney to collect. Collections can go after assets.
I would consider 3 or 4 things: (1) contact an attorney to collect the judgment(s), (2) contact DSHS/DCS to seek full enforcement services, (3) contact an attorney to file for contempt to establish judgments and fees/costs judgments, and (4) make sure all past due amounts are "linked" or "interfaced" with IRS for tax refund interception, vehicle license suspension and other remedies.