Withholdings

I have always claimed my children on my taxes and received refunds.  Now my youngest is working, and I cannot claim her any more.  I had no idea what to make my withholdings with my employer, I am single with no dependents now, and when I filed my taxes, I owe like $1600.  I have no idea how I will pay that, I don't make much, but how can I adjust my W-4 so that this doesn't happen again?  I am terrified that this happened, and am literally at the end of my rope.  I set up a payment plan with the IRS, but as it is, I barely have any money left over at the end of my check.  I am seriously scared at this point.