Minor Possesion of Alcohol

I live in California and I am 19 years old. Last night I picked up a few buddies from work, including one who is over the age of 21. Since my buddy has his license suspended for an unrelated reason, I agreed to drive him to the store to pick up a keg for his BBQ the following morning. While en route to his house a Police pulled me over for "Having a brake light out". I really didn't think it was a big deal. I thought I'd get a "fix it" ticket but that wasn't the case. The police officer spotted the keg in the back seat and took my information. He asked for a receipt for the keg and ID of the person who bought it. They both checked out but he the police said I was unable to transport the keg since I am not of age. We had nothing else in the vehicle; no tap [for the keg], nothing illegal, NOTHING!
The officer did explain to me that since it is my vehicle I am responsible for anything and everything in the car. The cop wrote me a ticket for Minor Possession of alcohol. I have absolutely NOTHING on my record. I have never even been pulled over. You can say I'm a good kid and I was at the wrong place at the wrong time. Of course that will never hold up in court. Therefore my questions are:

-Even if my friend is of legal drinking age and he took the blame for his own alcohol in my vehicle, why do I receive a ticket and a court date?

-What are the usual punishments/fines/classes that I will have to have to endure?

And is there any advice anyone can give me? It would be much appreciated.

What section(s) were you cited for?

- Carl