Should I plead not guilty?

I have been charged with minor in possession of marijuana. When the cops pulled up my friends and I were sitting in the tailgate of my friends truck. The cops talked to us and then asked us if they could search the vehicle. We agreed. They searched and found a joint. Since the weed wasn't mine, I wasn't in the vehicle, and they can't prove that I was ever associated with the weed or that I had knowledge of the weed, how can they charge me for this? This was also a misdemeanor charge.