You should consider getting an attorney to contact the collection agency on your behalf to read and advise you regarding the civil demand.
Next, while you could have a public attorney represent you in the criminal court, you should consider having a consultation with a private attorney to help you decide if you would be better off fighting the case.
Note: For informational purposes only.
Depending on the court, a judge may assign a public defender for arraignment only and order you to obtain private attorney. He will have to make a finding that you can afford to pay for legal services.
In Nassau County, where I practice regularly, judges will not allow a 17 year old to appear without a parent and then their parents get assessed as to whether they can afford a lawyer. But if no judge brings this up , then you should be OK. The civil demand is one of the biggest scare tactics in law and in 20 years of practice I have never seen a client sued under it. I would hold on to it until the criminal case ends and then wait to see if they contact you again (Which I highly doubt)
You will need to have an attorney present before a judge will assign a public defender in Nassau County. Moreover, unless your parents are below the poverty level, they will be responsible for hiring a lawyer to represent you if you show up without one.
As for the civil demand, I always fight these on behalf of my clients, particularly if they are not convicted of the underlying offense (petit larcency). Often, firms trying to colllect the civil demand are playing on people's fears and vulnerabilities.
I'd suggest you contact the D.A. office handling your case and make them aware of the document requesting payment you've received. Ask if you paying the requested amoujnt will suffice as a civil compromise. If they say "Yes," get it in writing and thren have them set a court date to accomplish it. If paying won't resolve the matter, and you won't gain anything by paying - will the injured party (Walmart) let the case be dismissed if you pay the requested amount? if so, it's worth it, providing the D.A. agrees in writing not to prosecute it -don't.