This is a question I am sure a lot of birds owners ask. When it comes to taming cockatiels, it is helpful to know the history of your bird. If you purchased your cockatiel from another private owner, it is a good thing to know what kind of relationship or living conditions your cockatiel had in his previous home. The reason this is important is because your cockatiel could bring some baggage along to his new home if he was neglected or ignored in his previous home. All of these factors are good to know when it comes to taming your cockatiel and understanding why he acts the way he does as you tame him.
If you purchased your cockatiel from a pet store, it would be helpful to know how long the store had your cockatiel, did he get social time with the pet store employees, was he handled regularly? Also it is good to know how old your cockatiel is. All of these help you understand your cockatiel a little more and ease the taming process.
If you purchased your cockatiel from a reputable breeder who hand raised your cockatiel, you job is going to be much easier. Cockatiels raised by breeders are typically already tame because they have been handled and trained every day. They will be easier to work with for you because you will just be focusing on getting to know each other etc.
The main thing to remember when you are working with your cockatiel is to respect his space and what he is feeling. If he is uncomfortable when you get too close to him, back away and try again later and until he is comfortable with you being close to him. If you force your intentions on him, you will be conditioning him to be afraid of you. This is called unintentional conditioning.
Taming your cockatiel may take some patience on your part, but if you use the proper training method it can be easier than you thought possible. And you and your cockatiel will bond and form a close companionship that will last for many years.