I was charged with internal possesion cop told me to plead not guilty in court what does this mean

All the officer meant is that you should go to your initial appearance and plead "not guilty." If you can't afford a lawyer, the judge will appoint someone for you. If you can, either call one before your initial appearance or hire one shortly thereafter.

The reason for this is pretty straightforward: after your initial appearance, the court will set a pre-trial. At that point, you can talk to the prosecutor and probably to the cop. You can arrange for a more favorable plea than pleading as charged at your arraignment.