Oh, I am so glad you asked as I have personal experience with this. With my second son, I potty trained in "reverse," starting with poop. He was 17 months, and I started doing it becasue one day he wanted to sit on the potty like his 3yo brother, so I let him, and for some reason, he pooped! I was so excited, and made a really bug deal about it (sang, danced, high-fived, waved bye-bye to the poopy, hugs and kisses). After that, when I would notice him about to poop in his diaper, I would rush him to the potty and he would go (followed by lots of praise). I didn't catch it every time, but I didn't worry about it. Within a few weeks, he was "telling" me when he had to go by pulling on his pants and running to the potty. If your son is already saying "ca-ca"...even better! I would say, whenever he tells you "ca-ca", take him to the potty. If he doesn't go within five minutes, wash up and be done and try again next time. When he does go, make sure to shower him with praise, even if he starts in his diaper and only finishes in the potty.
I really didn't focus on it too much, or push the pee-pee part right away. I saw it as more of a process, or a stage in the process. And, at least I never had to change another poopy diaper! The pee-pee part just naturally kind of followed over the next few months, and he got the idea. Yes, it did take about six months, but the upside is, he was completely potty trained by age two (yes, even at night). I guess I am more of a "slow and steady" person, rather than being in a rush to have it over and done with.
Good Luck!
BTW: I did keep him in pull-ups until he was staying dry all the time. I guess it could have happened quicker if I didn't (as other moms are saying). But the upside to that was, I never had any accidents to clean up.