It's my opinion that that anytime you are ordered to appear in court you should be represented by counsel. You do not want to say or do anything that will result in a situation where you would forfeit your right to have your record sealed. A lawyer can speak for you, without your being put on the spot.
Best of luck
It's always good to have a lawyer. There are so many issues in local court proceedings that can make for bad results. I have 9 years of college and stay up on my subjects of law every week. Congratulations on havering a bright future where you'd want a clean record. It won't be expensive to hire an attorney to make sure all is in order, and to be there for you in court. It will well be worth knowing a professional is with you.
This answer is provided as a public service and s a general response to questions. It is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
Had the court not ordered you to appear, you might have moved forward alone. Now: YOU ONLY NEED A LAWYER IF YOU WANT THE REQUEST TO BE GRANTED. The court will undoubtedly ask about the new conviction. Of equal importance is the matter that brought you before the juvenile court as well as your performance on probation. An attorney can help you prepare. Having any of your old probation reports or court records will help him/her to prepare you. While the infraction should not prevent you from getting your record sealed, the way you handle yourself (or better still, the way your lawyer handles the appearance) can make or break the deal. Hire an attorney who practices in juvenile court; if that lawyer is familiar with the court (Santa Cruz?) where you will be appearing, so much the better.
You must be present if ordered to do so. You should get a lawyer so that he can present the strongest case on your behalf. The judge will likely question you about the petty theft and determine if you have sufficient change in character that warrants having the juvenile record sealed.