I agree that it does matter if the judge promised you a particular sentence. You do need to find this out before you consult another attorney, but do consult another attorney if you don't feel that yours was working out for you.
This is a serious charge, a felony. There could be quite a bit of jail time involved for this and no one should go to court on a charge this serious even as a juvenile without an attorney. There is some flexibility in the juvenile system, however, a felony is probably some jail time, depending on any prior record. Definitely contact an attorney with experience in juvenile criminal matters as soon as possible.
You should always behave respectfully in court. The burden is on the prosecutor to prove that you actually shoplifted these items or just hadn't paid for them yet. As stated above, you really should speak with an attorney and discuss all the facts with them.
One thing to understand is that warrants do not go away until there is some satisfaction by the court. An attorney that is from the area and knows the judges could help make this go much more easily for you than if you go alone, but you should go in voluntarily.