You might be eager to get your child into underwear so you can avoid the cost and mess of diapers, but that doesn't mean your child is ready. Children should reach certain developmental milestones before you potty-train them. The surest path to potty-training success starts with your child being ready on all fronts.
Before they can potty-train, children need the ability to tell when they need to use the potty and hold it until they can get to the potty. This happens at different ages for different. Staying dry for at least two hours at a time is a sign that your child is ready for this step.
A child also needs to have a certain amount of gross- and fine-motor skills in order to use the potty. Those skills include the ability to get herself to the potty, which might include walking up stairs, standing on a stool or opening doors, depending on the setup in your home. She should also be able to pull her pants up and down by herself, though she might ask for help in emergencies. Young children should get some help with wiping for sanitary purposes, but your child might be able to give wiping herself a try.
A child needs to have a certain desire to be independent in order to start potty-training. He might, for example, want to be seen as a "big boy" or actively seek out the praise of others. It's at this point that rewards such as stickers or the ability to go to preschool become effective. Some children prefer to be babied a bit longer and those children might not be ready to start potty-training.
To be ready to potty-train, children should have the social awareness to see that others use a toilet rather than a diaper and understand that this is something they should be working toward. Seeing other children slightly older use the potty can help them realize that getting out of diapers is an important step.