Now the chance of drawing all the tiles you need to complete your hand directly is relatively low (though still doable). That's why the game will also allow you take the tiles discarded by other players and add them to your own hand. The requirements for that to happen vary depending on what tile you're going for. Here are the different steals and their requirements:
Chi
You can call Chi when the player before you (it doesn't work for the player after you or on the opposite side of the table) discards a tile that would allow you to complete a sequential meld. For example, if you had a 3 man and a 4 man in your hand you could call Chi if the player prior to you were to discard a 2 or a 5 man.
Pon
You can call Pon if any other player (this one isn't exclusive to the player before you like Chi) discards a tile that lets you complete a meld of 3 identical tiles. For example, you can call Pon if any player discards a red dragon tile while you already have two in your hand.
Kan
While most hands are made up of melds of three tiles there's an exception to that rule. You can call Kan whenever you have the option to form a group of 4 identical tiles. This works when any other player discards a tile that you already have 3 of but you can also do it when you have all 4 tiles in your hand. Calling Kan will reveal an additional Dora tile (Refer to Scoring for more information on Dora tiles). Quadruplet tiles assembled by calling Kan act the same as a triplet, so you'll not have a total of 15 tiles in your hand.
Ron
You can call Ron when you have the option to steal a tile that lets you complete your entire hand. Calling Ron will result in the player who discarded the tile in question having to pay for all your winnings in that round (your winnings would otherwise be split up between all other players).
Stealing a tile from another player may seem extremely tempting at first, but more often than not it's a better idea to show restraint. When completing a meld through stealing you will receive less points than normal for that specific meld (further elaborated on in Scoring) and you will no longer be able to call Riichi (further elaborated on in Scoring). You will have to rely on your own judgement on if that steal is worth the consequences or not. Here's a few things to keep in mind when making that decision:
Worth of the meld
Some melds will get you more points than others. For example, a triplet of three dragon tiles will get you so many points it's almost always worth it to complete it. On the other hand, a simple meld of three sequential tiles will usually not get you all that many points.
Chances of completing the meld
You can obviously only complete a meld by using the tiles that have not been played yet. Look at your opponents and their discards. If the tile you're looking for is a 2 sou and you see that your opponents have already discarded 3 of the 2 sous in play that would mean that there's only one tile left in the game that could complete your meld. In that case you might want to grab it at any chance you can get.
State of the game
The longer the game goes on, the more likely your opponents will be to be close to finishing their hands. If that happens you won't get any points for that round. So if the round has been going on for some time and your hand isn't looking too good you may want to complete your hand at any cost and steal whatever you can.
Do you even need that tile?
Self-explanatory, but if you've already got a complete meld and the game asks to you steal a tile to complete that meld there's no reason to do so.
For example, imagine that you have a 3-4-5 pin in your hand. The player prior to you discards a 2 pin and the game asks you to call Chi to steal it and complete a 2-3-4 pin. This would only leave you with a meld of the same value as what you already have while giving you all the disadvantages of stealing a tile. Don't do it.
