If you're planning to camp at Yosemite next November 15 through December 14, be ready at 7:00 a.m. Pacific Time on July 15 to make your reservations online. You can do that online or by phone at 877-444-6777 or 518-885-3639 from outside the US & Canada. Get an overview and more options here.
July is also the time to start thinking about California State Parks camping reservations for next February. On August 1 at 8:00 a.m. Pacific Time, they'll release the entire month of February at once. Everything you need to know is here.
Now is also a good time to firm up your thinking about whether (and when) you want to visit one of the camps in the high Sierras next year. The lottery to get a reservation at one of them will start before you know it.
This tip is in case you didn't plan ahead: If you're looking for a last-minute room in a popular spot, especially at Yosemite, check the hotel website to find out the last day of the free cancellation period and try calling several times that day.