First slice off the top of the onion (leave the root on for the moment), then remove the papery skin and any brown outer layers.
To chop the onion, cut in half from top to bottom. Put the cut side down and make a number of horizontal cuts towards, but not quite reaching, the root. Then make as many vertical cuts through the onion, again not quite reaching the root. Holding the onion very firmly and with the knife blade at right angles to the first set of cuts that you made, slice down vertically - the onion will fall away in small pieces as you go. Continue cutting until you reach the root, which you can now discard.
To slice, trim the root off, then cut in slices moving from the root end towards the top. Leave as slices or separate each one out into rings.
All onions are best prepared just before you use them.
Read more about onions here
All the best Jan