Canasta is a card game for two to six players, perhaps best with four players. It also has widely varying rules, mostly based on region. So, for this description, I will stick to the most standard version, the one described by Hoyle and accepted by the National Canasta Laws Commission.
Rules of Canasta
Players: As stated above, Canasta is playable by two to six players. It is best played with four, as this balances the game very nicely. With two or three players, each player plays for his or herself. With four players, it is played with two teams of two. With five, it is played again with two teams of two, but one of the teams rotates a third player in and out. With six players, it is played with two teams of three. Canasta can be played with more, but it is much less enjoyable.
Cards: The game is played with two standard decks of fifty-two cards, plus four jokers, making a total of 108 cards. These cards are shuffled together. The jokers and deuces (those with a face value of two) are wild. If you are playing with six players, it might be fun to try it with three decks and six jokers.
Before the game: Shuffle the deck and have each player draw for partners; highest card chooses the seating. The player drawing the highest card also plays first, meaning the player to his right deals first. After this, the deal passes to the left.
Dealing: The dealer gives eleven cards to each player one at a time in a clockwise or counterclockwise fashion, beginning with the opponent to the left and ending with the dealer. The remainder of the deck is placed face down at the center of the table to form the stock. The top card of the stock is turned face up beside it; this is the upcard. All subsequent discards are made on the top of a pile started by the upcard (if the first player doesn't take it). Only the top discard may be seen.
Red threes: If the upcard is a red three, then it should immediately be covered by the next card in the stock. Also, if any players draw a red three in their opening hand, they must lay it face down in front of them and then draw a replacement card from the stock. A red three taken in from the discard pile is also always laid face up, but isn't replaced from the stock.
Playing the game: The player at the dealer's left (always an opponent since partners sit opposite each other) plays first and thereafter the turn passes to the left. Each turn comprises a draw, the option of creating or adding cards to a meld (as many times as you wish), and then a discard of a single card.
The draw: When a player's turn starts, he/she may either take the top card from the stock and add it to his/her hand (or lay it in front of them as with the red three rule). The player may also take the top discarded card and immediately add it to a meld that already exists, then add the remainder of the cards in the discard pile and add it to his/her hand or to melds that already exist.
Melds and melding: The goal of the game is to form melds, combinations of three or more cards of the same rank (sequences aren't valid melds in canasta). It must contain at least two natural cards of the same rank (meaning non-wild cards) and no more than three wild cards. Also, black three's cannot be melded unles the player goes out on the same turn. Jokers and deuces cannot be melded by themselves; they are wild cards and violate the restriction on wild cards in melds. A meld must be laid face up on the table for all players to see. A player may build on a meld by adding cards to those owned by him/her or his/her partner (NOT an opponent's meld).
Canastas: A meld comprising seven or more cards is called a canasta and really helps to cinch the game. A canasta is worth 300 bonus points. In addition, if the canasta meld contains seven or more natural (non-wild) cards with NO wild cards, it is a natural canasta, and is worth 200 more points (making that canasta worth a total of 500 points). Canastas swing games, period.
Minimum count: Every card in every meld has a point value, as follows:
joker ... 50 points
deuce ... 20 points
ace ... 20 points
king, queen, jack, 10, 9, 8 ... 10 points
7, 6, 5, 4, black 3 ... 5 points
The first meld made by either team is its initial meld. The initial meld has a minimum count (i.e., a total point value) that this initial meld must have before it can be played from a player's hand. Canasta bonuses don't apply. The minimum counts are as follows:
If the total score of the team is less than zero, the minimum count needed is 0.
If the total score of the team is between zero and 1495, then the minimum count needed is 50.
If the total score of the team is between 1500 and 2995, then the minimum count needed is 90.
If the total score of the team is 3000 or more, then the minimum count needed is 120.
The purpose of this is to give a team that is behind due to bad deals something of a chance to catch up. Once the initial meld is made, then additional melds can be made without any minimum count.
Concealed hand: If a player can go out without having played a single card up to that point, that player earns his/her team 100 bonus points.
Ending the game: The game ends when one player has no cards in hand (going out) or there are no cards in the stock and the player refuses to take the top card from the discard stack. If a player can go out before drawing, he or she must have the permission of the partner, and then that player can skip the draw and play all of the cards in his/her hand.
Scoring: The team that goes out calculates their score as follows:
total value of all cards in that team's melds
+ 100 points for going out
+ 100 points for each red three
+ 400 points if that team has all four red treys
+ canasta bonuses and concealed hand bonuses
- total point values of all cards left in the hand of the player that didn't go out
The other team calculates their score the same way (without the "going out" bonus), except that if they did not have a meld, they lose their points for the red threes.
A running total of the scores are kept. The first team over 5,000 wins; if a team goes over 5,000 mid-game, the hand is still finished. If both teams are over 5,000, the team with more points wins.
Basically, the most effective strategy is to pick up the discard pile early and often in order to make many big melds. My advice is that if at any point you can pick up the discard pile, do so; more often than not, it will help you out. Another point of advice is to not make the initial meld until you've picked up the discard pile a time or two, especially if you only need a value of 50 to play the initial meld.
Two more points of advice. Try to make regular canastas rather than waiting around for natural ones; you risk your opponent going out, so get what bonuses you can. Another idea is to, if you can, keep your score below 1500 or 3000 for another hand, even at the expense of a few hundred points. Being able to play the initial meld early often makes a huge difference, letting you get out the door before your opponents can do anything.
Canasta is a very fun and fast paced game, definitely most enjoyable with four players. It builds up tension nicely, as it becomes more challenging as your point total nears 5,000. It also plays pretty quickly, often times in less than a half an hour. It's good, clean fun that exercises your mind a little.