Cockatiels are very friendly birds and tend to be easy to win the trust of. Once you have their trust, they also tend to seek out human companionship. This, combined with a generally easy going nature makes cockatiels comparatively easy to hand train.
The most important thing in winning the trust of a bird is patience. Birds will do things on their own time, and usually that will take a while.
When you first bring your bird home, he or she will normally be very frightened by the new place and new people, so before you start to work with your bird give him or her two or three days to calm down and get used to their new surroundings.
Next, you will need to find a "neutral" area to work in, one that the bird has neither claimed nor is afraid of. The fewer disctractions the better. Some people like to use a bathroom, but you will need to make sure that the toilet is covered and if you think the bird might be startled and fly, cover the mirror and windows as well to prevent any injury. I use a small guest room with mesh over the window. This room should be completely out of sight of the bird's cage or any other birds.
Depending on where you got your bird from your bird will be at a different stage of trust with humans and will need a different strategy. Many birds adopted from a good shelter and babies bought from a good breeder will already trust humans and may only need to get used to their new home and new people. A cockatiel who has never had any positive human contact may take months to finally trust humans. A cockatiel who was once abused may never trust humans again, or only trust people who don't look like to person who abused them.
Step One
Getting closer to your bird.
The first thing you need to do is let the bird get used to you being close to him or her. This part is both the easiest and the most frustrating because there is little you can do to speed it up and it may take a while. Just sit close to your bird and talk to her. Say anything you want. Tell her how your day was, read a book to her, or better yet, tell her how beautiful she is. Parrots are very vain creatures and love compliments. Watch your bird closely. If she raises her crest all the way up and moves away from you, scoot back. If she is hunched over with her crest pressed flat, swaying or hissing she is afraid and ready to fight if she needs to, so scoot back and slow down. If her crest is up half way and she is looking at you, she is curious about you and not sure whether to be afraid or not, so stay where you are. If she moves closer to you with her crest all of the way down, you can move closer to her. Keep this up for a few minutes every day, twice a day would be better if possible, until your bird is comfortable in your presence enough to look away or blink while you are looking at her.