How can you tame your cockatiel?

The process of taming an adult parrot involves many, many things that can't all be put on this website. One would also need to have many, many more details from you on your particular bird as well as yourself in order to provide this information. I would recommend you research the many, many websites on the web, read bird-related magazines, etc., and/or find someone close to you (such as a breeder or other knowledgeable bird owner) to help you with this. The answer to your question isn't as simple as a short paragraph!