My ideas on potty training are a little different than the average mom these days, but I think my results speak for themselves - my kids were potty trained by 2 years old.
When is the kid ready? When you say they are. Let's face it, potty training is like teaching any other skill - eating with a spoon, putting shoes on, learning to swim. You, the parent, have to be committed to teaching this skill 100%. You need to give your child clear, consistent, cheerful direction over and over and over and over until they have mastered the skill. That is all there is to it. Yes, you can wait until they're 4 and they magically figure it out on their own, but I personally think it's mean to make a child poop on themselves until they figure it out on their own - especially when I have the knowledge to teach them properly.
Now, as for the logistics of potty training. Begin on a day when you don't have anything outside your home that you need to do. (You will need about 3 days at home, ideally.) If possible, have your child naked from the waist down all day (no underwear, no diapers). Have your child sit on the potty (or potty chair, whichever he prefers) every 20 minutes. (Set a timer - this will seem like an insane frequency to be doing this, but stick with it.) Sometimes he will go, sometimes he won't. If he does, celebrate like crazy! If he pees when he's not on the potty, clean it up calmly and tell him, "You will make it to the potty next time." And then make sure he does.
Now, during the first day, he will have more mistakes than times he gets it right. You will think to yourself, "WHY am I doing this? He will never get it! He's not ready!" But don't think that. He is learning a new skill, and Rome wasn't built in a day! Stick with it and keep encouraging him. When he begins to understand what you expect him to do, and he has some successes, things will start to get easier. By the end of day 2, you will be doing a lot more celebrating successful pee and poop in the potty. By the end of day 3, you should be able to move to him wearing underwear. Yes, he will still have some accidents - keep reinforcing, reminding, encouraging and teaching him.
As a side note, he may or may not be able to handle wearing underwear at night. Night time potty training is completely different from daytime training. If he wets the bed at night, just put diapers on him until he's able to stay dry. One of my girls was daytime and night time potty trained at the same time. The other one had trouble with night time dryness until she was 5. So, that will either come or it won't, not much you can do about that.
Anyhow, I hope this helps. It worked for M. twice, and it is the method my grandmother used for all 7 of her children, and what my mom used for M., so this method has a good track record! ;)