At an arraignment hearing, most people charged with a criminal offense enter a not guilty plea. This allows more time to acquire an attorney, review the police reports, investigate the charge, and consider possible diversion options or sentencing recommendation of the prosecutor. Once more information is provided, a person may decide to enter a guilty plea- typically at a later pre-trial hearing.
If you plead guilty it means you admit the charge and elements to prove the charge. By pleading guilty you waive your constitutional rights and in most cases will be sentenced right then. However, you may speak on your behalf at sentencing. The Judge will then usually review the police report, if available, and sentence you.