As executive said, get an address first. We got one from Goodsam Mail in Crestview, FL. Has worked great so far. Then we purchased our rig and had the dealer register and title it in Florida. You will pay FL sales tax, not CA.
Once you have a vehicle registered in FL, it is really easy to get a DL there, using the registration as proof of address. We also changed our bank account address just to make it easy.
We bought the vehicle after we sold the house but before we went to settlement. Because we owned a house and were not full timers, our interest rate was 1% less and the loan was easier to get.
At the dealership, we went online to Progressive.com and bought insurance. It was incredibly easy and took all of seven minutes. We got the much much cheaper Crestview rates because we were registering our vehicle there.
About a month or so later, after settlement, we went to FL and got drivers' licenses.