Pleading not guilty doesn't affect your later ability to change your plea. Pleading guilty will result in your being convicted and sentenced.

If you qualify for a court-appointed lawyer, consider petitioning the court to appoint one for you. You would benefit from having your case reviewed by a local criminal defense lawyer, who should have a pretty good idea of what a prosecutor is apt to offer by way of a plea bargain and whether you might be able to avoid a criminal conviction.