Criminal record... How can it be expunged?

I am a college student who commited two petty theft at school in California. One was at the bookstore, when I got caught no police was there but the bookstore sent the report to police. Is this considered as an arrest and conviction? Another case I got caught stealing a book at the library. This time they called the cops, then the officer wrote a report but did not take me to jail. Is this considered as an arrest and conviction? I am very worried about my criminal record. How can I get these records expunged? Do I need an attorney to do so?