Look at what your player has melded (exposed cards) and avoid throwing them out. Keep 3s for the very end as it prevents the other player from melding and hence taking the entire deck. 2s and jokers are wild cards use them to form mixed canastas.
If you want to win canasta, you have to get rid of all the cards in your hand. First you need 3 of a kind to lay them down. When you have all 6 cards, you get a canasta and 100 points.