Having a MMJ card does not exclude you. The cards last a year. You could stop using, and then fill out the 4473 without lying. I would suggest making sure you could piss clean first before buying a gun.
Secondly, many departments in CA are time based. Some as short as a year, some as long as five. Be grateful, as other states would auto DQ you for this. As was said by Mr. Solo, do activities to show maturity, do well in school, and ensure your associations are as good as possible.
I am through this process myself. I am unable to do any volunteer work with San Bernardino Sheriff until next May (volunteering at ThinkTogether and soon to be Probation in the meantime); however, the backgrounds investigator I spoke to was relieved that it was -just- marijuana. Do not let the wives tale of "you have to compete with people who have never used drugs" fool you, as pretty much everyone has done something nowadays. The investigator even mentioned to me that they scrutinize liquor more now than they used to, so no one is squeaky clean.