For humans, a diet of fresh fruits and vegetables is a good thing. For your pet bird, some of those things that are healthy for humans are toxic for birds.
DO NOT FEED YOUR BIRD:
Avocados, for one. Do not give your bird avocado in any form. There is an enzyme in the pit that can leach into the fruit and be perfectly fine for humans but toxic for birds.
Do not give your bird onions; there’s too much sulfur in them for a bird to digest.
Avoid garlic; it has similar chemical compounds to onions.
Do not feed your bird tomatoes; they are too acidic.
Also, avoid mushrooms; they are a fungus that birds cannot digest well.
Finally, stay away from celery, specifically, the strings. Birds (except parrots) cannot chew the stringy fibers well.
Citrus fruits. Again, too acidic.
(Also, it should go without saying, but no chocolate, alcohol or caffeine either.)
There are still plenty of healthy foods you can give your bird.
DO GIVE YOUR BIRD:
Apples, grapes and bananas. These are easy to chew and digest, and they supply lots of vitamins for your bird. Cut into a small dice before giving to your bird.
You can also give corn, broccoli, carrots, yams and peas. Your bird will enjoy the different flavors, colors and textures.
When in doubt, ask your vet!