how do u know when onions r ready?

Jane; assuming we are talking about onions from sets then: the tops should start to turn yellow soon and wither. There is nothing to stop you pulling them up now and eating as many as you need, thats what we do.

If you want to store them you need to wait till the tops have turned brown then pull them all up and lay them on netting or something to raise them off the ground so they can dry out for a couple of weeks. Then they can be stored in a dry cool dark place, they will keep till next year.