Potty training is a pretty loaded subject - and pretty much everyone will give a different answer, since every child does it a bit differently.
I think potty training is the first opportunity to allow your child to figure things out on their own, mostly, or it can be the first time power struggles really heat up between parents and their children.
In our current culture children are being asked to start potty training earlier and earlier. Since every child develops differently in regards to when they are aware enough of their body to start potty training, the "when" question to me really is a question of personal body awareness. You'll meet some (extremely few) babies who are using the potty reliably at 1 year old, and you'll meet others who are just getting to the reliable stage at 4 years old.
Body awareness means that they are conscious of when they are going to the bathroom, they know where it comes from and can predict the outcome of the tickle or pressure they feel in their body to producing urine or a bowel movement. Children who are potty training are almost always able to verbalize what they are experiencing, and have simple words to associate with the happenings in the potty or their diaper.
Parental pressure vs. parental support:
It's easy to get overbearing when it comes to potty training as a parent. But if you haven't noticed already, pushing your child to do something generally results in the pushing back by the child in the opposite direction. So how can you gently introduce the idea of the potty without using pressure and only using support? Start by talking.
1. When changing diapers, talk about what is in the diaper, and their body parts. Choose the language you want your child to use, and repeat, repeat, repeat.
2. Observe your child's behavior around diaper changes. Are they responding to what you're saying? If there's no response, moving ahead is probably not going to do anything but create frustration. If they're mimicking, touching, and talking back - then it's time to start the next step.
3. Observe your child's real time life behavior - are they touching their diaper and starting to use body or verbal language to show their awareness of activity? Most often, they will do a little dance, or touch, or look for privacy BEFORE they actually produce anything in their diaper. This is the time to gently ask if they are going to go pee or poo (or whatever your language is). The only purpose of this is to start helping them understand the process of pee or poo - they feel a sensation, and it means that they are going to make something in their diaper. At this stage it is not appropriate to rush them off to try and sit on the potty. That's too much too fast.
4. Introduce the potty. Some families choose a child size potty, others just put a special chair so the child can reach the regular potty. Either is fine, however some children need to re-learn how to use a big potty down the road if it hasn't been being used all along. The easiest way to introduce potty and keep positive for me has been to have the child come in the bathroom with you. Then, as you're using the potty, you narrate what you're feeling, and doing. If you're uncomfortable with this, I would suggest using a book that shows the real process, and reading it daily. Let your child know in the beginning that they can use the potty too, instead of their diaper. Remember to phrase it as a choice.
Depending on the age of the child, you can also say things like:
"When you're a big kid, you'll use the potty all the time, and you won't use a diaper anymore!"
"Mommy used the potty when she was three years old. Do you think you'll use the potty when you're three?"
"The potty is great! It keeps your skin clean and you won't have to do diaper changes anymore."
Keep all statements positive, shame and competition free. Beware of using statements too often - this will come across as pressure, instead of support.
5. When the child shows interest, is responding to their body, and feels comfortable exploring the potty, ask them if they'd like to try. That's all. Just ask - no telling, no pressure, just a simple question. Also, make sure their "no" is heard loud and clear. The response to a "no" might be "Okay, when you're ready, you'll do it!" In a light tone. Then change the subject quickly.
6. Provide fun potty related activities. A special book only for reading on the potty for when they're ready to sit on it is perfect.
7. Avoid rewards. Have trying something new and getting a positive response be the reward. No M&Ms, no special treats. Do you really want food and using the potty to be tied together in your child's neurology? Potty use can be fun, it can be exciting for parents to see the first successes, but it really is just something you want them to just do. It won't be a big deal for long if it's taken in stride instead of it being a big opportunity for drama - even positive drama!
8. Peer pressure. If possible, having your child hang out with other kids their own age or a bit older can do wonders for getting them excited about potty. Using "heroes" has also done amazing things. Elmo has a great potty video, have a hero show up a the door in costume (superman always uses the potty, and he leaves behind a special potty figurine of himself just for Ben). Talk about how the dog goes outside to go potty, or how the cats use the litter box. Talk about how the mail carrier, the people they see on a walk, Grandma and Grandpa - they all use the potty.
9. If you're in a time crunch: Daycare is often a great motivator for parents to get their children potty trained. I sure hope you aren't in a time crunch, as most potty training takes a solid year to really master. If you really are and have no choice, then using heroes, and rewards (not food, please), buying special underwear with their heroes on it (yes, that's why underoos were created) or anything else you can think of might help.
10. Remember that potty training can be fun, if you keep it that way. Accidents are just that - make sure to keep your temper, even as your child has a huge wet accident in the pew at church! While potty training don't expect instant success. Allow your child to wear those fun new undies over her diaper, or let her know that using the diaper is okay, too - when you're out and about. At home, you'll try to use the potty, and accidents are okay.
11. Don't be surprised if pees are consistently done on the potty long before poos ever are. It's very typical to put a child in a diaper for the night, tuck them in, and then have a poo that needs to be changed 5 minutes later. Again - no shame, just keep it light and loving.
12. Setbacks are usually related to stress, illness, or pressure.
Best of luck, parent of a future potty user!