There are probably many things you can do, but I would suggest it comes down to one main thing, which is gaining the bird's trust. There is every chance that the bird is hostile because it is not familiar with you. If the bird is fairly old, then you may be fighting a losing battle. However, we should look to the positive side first. Try to spend lots of time talking to the bird while he/she is in the cage. Try not to stick your hand inside the cage if you are letting it out. The cage is his home and he will think your hand is a threat. Same goes if you stick your fingers inside through the gaps. Also, try not to have the bird startled. Sudden movements will put your relationship with the bird back a fair way, especially if it is physically hurt from the experience. If you see no improvement after a month or so, try getting in contact with some breeders or an aviary specialised vet. That is not to say that he will be getting up on your finger or nibbling at your ear after a week, but you should see some improvement. Remember that cockatiels are sociable birds and will look for someone to be with. They just need to know that you are not going to hurt them.