Quick answer
On your turn you either claim the tile the previous player just discarded, if you can use it for an exposure or for mahjong, or you draw the next tile from the wall. Then you discard one tile face up and say its name out loud, which brings you back to thirteen tiles. Your turn starts the moment you touch the wall tile, and from then you must keep it and can no longer call the discard. After you discard, the other players can claim your tile until the next player racks or discards.
A turn sounds like two steps. In practice it has several, and the small ones are where table arguments come from, such as when your turn actually started, whether somebody still had time to call, and whether a tile counted as racked. Here is the sequence, in order, with the moments that matter marked.
The short version
Play passes to the right. On your turn you either take the tile the previous player discarded, if you can use it and claim it in time, or you draw the next tile from the wall. Then you discard one tile, face up, and your turn is over.
You hold thirteen tiles between turns. During your turn you hold fourteen, briefly, which is why every turn ends with a discard. The discard puts the count back to thirteen.
Your turn begins when you touch the wall
This is the detail that settles arguments. Your turn is considered started the moment you touch the next tile in the wall, and from that moment you must take that tile. You do not get to touch it, think better of it, and put it back.
It has a second consequence. Once you have touched the wall tile, you may no longer claim the discard you were still thinking about. Reaching for the wall is itself a decision not to call.
The sanctioned tournament rules state this plainly, and it is worth knowing at any table, because it is the rule that decides who was right when two people move at once.
Pick, pause, rack
Take the tile, look at it, then place it in your rack. The pause between picking and racking is not dithering, it is the window in which an opponent can still claim the discard sitting on the table.
Picking and racking in one fast continuous movement is called out in the tournament rules as unsporting, because it denies the other three players a fair chance to call. Players are asked to leave a beat between the two.
A tile counts as racked when it is placed inside the rack, not balanced on top of it and not sitting in front of it. That definition exists because racking is the moment that closes other people's options.
Name your discard
Say the tile out loud as you discard it. This is part of how the game works rather than a courtesy, because the other players are entitled to hear what went out, and a discard named wrongly has its own rule.
A tile counts as discarded once it is named or once it touches the table. Either is enough, which is why a tile you have named is gone even if your hand is still on it.
The claim window, and when it shuts
After you discard, the other players have a window to claim that tile for an exposure or for mahjong. The window stays open until the next player either discards or racks the tile they drew.
It also shuts permanently once the discard is covered by another discarded tile. A covered tile cannot be claimed by anyone, for any set, including for mahjong.
So if you want a tile, say so immediately. Once the next player racks or discards, the rule gives you no way back.
Drawing early, or from the wrong end of the wall
Drawing your tile before the player ahead of you has discarded is called picking ahead. It gives up your right to call that discard, and the standardised tournament rules forbid it outright. Under those rules a player who picks ahead, or picks out of turn, is declared dead. Tables differ on picking ahead in casual play, so ask before doing it at a new one.
Drawing from the wrong end of the wall, or from the wrong wall, makes a hand dead under the same rules, and that player stops picking and discarding. If the mistake is not noticed until more than one player has drawn from the wrong place, the game continues and play goes on drawing from that wall. When you see an opponent reach for a tile out of turn or from the wrong end, the rules call it polite to warn them before they look at it or rack it.
If you want a joker from somebody's exposure
You may swap the real tile for a joker sitting in an opponent's exposure, but the order matters. Do the ordinary part of your turn first, meaning pick from the wall and rack, or claim a discard and expose, and only then make the exchange.
Exchanging before you have picked is the mistake that gets a hand declared dead. The sequence is not decorative, it is what keeps the tile count honest.
The things you cannot take back
Touching a wall tile, which commits you to it and ends your chance to call. Naming a discard, which sends it whether or not you meant it. Touching a discard you claimed, which commits you to using it in an exposure. Exposing a set, which is public for the rest of the hand. And a joker exchange, once your tile has gone to the other player or you have taken hold of their joker.
Everything else on your turn is reversible, which is why the useful habit is to decide before you reach. Look at the discard, decide whether you want it, and only then move your hands.
Put it into practice
New to the game? Start the course at Lesson 1, Meet the Tiles. The first three lessons are free and need no account.
Already play? Have the coach read a real dealt hand for you, free. It names the direction the hand goes for, what to keep, what to pass, and the reasoning behind each call.
Turn order is the part you stop thinking about after a few sessions. What stays hard is deciding what to discard, every single turn. Mei, our AI coach, reads a dealt hand and says which direction it goes for and which tiles are doing nothing for it.