i have grown onions they have like pods on the top when are they ready to dig up and use

If your onion plants have flower buds, which is what the pods are, then they are about to go up to seed. If the onion is ok then harvest it immediately. If you leave it, the plant will flower and the onion will be useless. Some people cut off the flowers, but I find once they've done this the onions are never any good. It's caused by poor growing conditions, alternation of drought and heavy watering. Onions are biennial so should form the bulb in the first year, and the seed in the second. What has happened to yours happens more often with onion sets than onions grown from seed.