Some birds never learn to talk. Other birds pick up words quickly. It is very individualized.
Generally, if you want the best chance at getting a "talker" you want to start young and have just one bird. Spend lots of time together and work on developing a strong bond. Talk frequently and be consistent. For best results, try to associate particular words or phrases with a trigger - like saying "dinner time" when putting food into the bowl or "want to come out?" when opening the cage door. Eventually, the bird might start to say "dinner time" when its bowl is empty and it wants you to fill it.
Even if your bird doesn't learn to mimic your speech, it might learn to understand your verbalizations, which can be useful for training purposes.