Why is your cockatiel not very tame?
Cockatiels are wild birds so if they were never tamed when they
were young, they will still be untame.
It is easier to tame a young cockatiel, however with the right
attitude, you can tame an adult cockatiel.
You have to start with trust. Taming a cockatiel take a while
and one wrong move could ruin all your work. Start with standing
next to the cage and talking to your bird. Do this for about a week
so your cockatiel gets to know your voice and is calmer around
you.
Then you could try putting a fresh vegetable (in your hand) next
to the cockatiel and hold it there. Don't force the cockatiel to
get closer to your hand - this will scare him/her. You need to earn
trust.
Eventually, your cockatiel will be comfortable with you having
your hand near him/her. If you cockatiel is comfortable to actually
eat from your hand, you can try picking it up.
Do not grab our cockatiel on its back, try to persuade him/her
to sit on your finger. Do not force your cockatiel to get onto your
hand. This will ruin the trust. Eventually your cockatiel will be
ready to sit on your finger and eventually your shoulder! It's all
about trust and if you're cockatiel trusts you, it would love to
sit on your shoulder!