You have the right to request a new attorney. You will have to show cause before the court why a new one should be appointed (for example, that your current attorney has made no preparation for defense).
If what you say is true, you may also wish to report the attorney to the State Bar Association (or relevant disciplinary body) for malpractice and misconduct. Your attorney is obligated to diligently represent your interests, even if he is court-appointed.
However, before taking such extreme measures, I recommend you sit down and discuss the case with the attorney (if you have not already). Express your concerns to him, and see if there is anything he has already done/will be doing to prepare the defense. Depending on the case, he may be giving you a fair assessment for prevailing.
If however, you truly believe that your attorney is not competently handling your case, you should do the following:
Create a log of all the times your attorney fails to do his duty. That is, if your attorney fails to return calls, keep you abreast of current actions in the proceeding, etc. This will be useful both in your request a new attorney, and should you feel it necessary to bring disciplinary actions.
Also, contact a local legal aid foundation (if there is one), and see if there is any who can refer a pro bono attorney.