What can I do if I have a bench warrant for petty theft for $50?

You should hire a lawyer to go to court to recall the warrant. A lawyer can do this without you present if the underlying charge is a misdemeanor, so it is less risky for you. Petty theft is a crime of moral turpitude, and if convicted it will really hurt your school/employment aspirations. More reason to talk to a lawyer, who may be able to work out a deal with the Judge whereby you can earn yourself a dismissal. But first, the warrant must be recalled.