Welcome!
Your wetsuit thickness will depend on how cold or not you become in the water. If you're always cold, you may need a thicker suit, or layers. When students (here in San Jose) are renting their wetsuits, they get either a 7/8 one-piece or a 7/8 farmer john. The farmer john actually gives you, like, 14 across your core and upper-thigh area. If you choose the one-piece route, you can choose a hooded vest. Again, it will give you something around 12-14 mm across your core. Some people find that wearing a "skin" underneath their suits help with heat loss AND getting the suit on/off. But I suppose one of the biggest things to keep in mind is fit. If the suit is ill-fitting, it'll allow water to enter easily, causing you to get cold faster. The shop where you buy stuff should be able to help you find one that fits well. You can also try a custom-fitted suit. I hear they are a bit more expensive, but are worth it (i hear).
As far as temperature goes down in Monterey it varies from 48ish-53ish.