Log onto the website of the Calaveras County Visitors Center. It will advise you on campsites related to your specific needs, desires and budget. You can call it toll free during office hours at 1-800-225-3764.
Book somewhere to stay, either by using the recommendations of the Visitors Center or by conducting your own research. Angel's Camp RV and Camping Resort, Camping USA and Gorp all offer advice on where you can camp.
Drive to the town, even if you have not made any reservations. During parts of the summer, local residents may allow you to camp on their land, and will have advertised the fact in their front yard. Ask in a local store whether any campgrounds are nearby. Many campsites and campgrounds may not be advertised online, meaning that you will not be able to book in advance.