A cockatiel is a very clever bird that hones a number of skills. They are super climbers and can use their toes to hold on and hang upside down they are quite the acrobats. Cockatiels can mimic simple words that they hear repeatedly and whistle tunes with precision.
A cockatiels speech is understandable, but not as good as its bigger bird relatives like mynahs and parrots. If you want to teach your cockatiel to speak you should start with simple words. It is a good idea to begin with 2 syllable words like 'baba', 'papa', 'hello' or similar. Do not try to teach new words until the first word you want to teach has been learned properly, it will take a lot of patience but it can be done with regular practice.
When a cockatiel is owned from when it is young it is easy to tame, this is especially true when you own a single cockatiel, they will want to communicate with you if you communicate with them. A lot of authorities will recommend you to have your cockatiels wings clipped if you are going to be hand-taming your cockatiel. You should always get a professional to clip your cockatiels wings, don't attempt it yourself.