Many people ask me how to teach a cockatiel to talk, and the answer here is just patience, patience and more patience.
Patience is Key
Have patience, and never raise your voice, scare or show violence to your bird, or he may withhold words he does know. Keep training your cockatiel regularly to keep your parrots mind stimulated. Reward your bird when it achieves, by saying Good Boy or Good Girl and give him a treat.
Handle your bird frequently and make him or her part of the family. Parrots of any species are very social animals. Watch the language in the house, parrots are also very good mimics.
Never teach your pet cockatiel to swear. Your bird may out live you and then nobody will want to look after it.
Keep your bird stimulated, and take it out of its cage for daily exercise. A small amount of television will also keep him stimulated. Try the cartoon channels, as your parrot will enjoy the different sound effects.
Always teach your bird to speak before you teach him to whistle. Whistling is easier for most birds to learn.
When speaking to your parrot, emphasise the word that you want him to learn. You can add some volume and dramatise the word and your parrot will pick it up faster.
A cockatiel has the brain potential of a two year old child, so it can definitely learn to talk with just a bit of extra time and attention from you.