>>713147767 (OP)
You have suits (manzu, pinzu and souzu) that go 1-9, and honors, made up of dragons (Green, Red, White) and winds (East, South, West, North) there are four of each type of tile. You have a hand composed of 13 tiles+1 draw, players take turns drawing and discarding in order to compose a winning hand made up of four melds and a pair*, containing at least one yaku, while not in furiten**. A meld is either a triplet (three of the same tile) a sequence (three consecutive tiles, e.g. 123, 567)***. A yaku is a trait that adds han to your hand, han will be used in combination with fu to determine your hand's point value, unless you have 5 or more, then it gets ignored entirely, doesn't matter since online does score for you. What matters is that you need one, learn riichi, tanyao and fanpai, they're the easiest ones, and focus on learning more down the line. If your hand is one tile away from forming a valid hand, you're in tenpai, this means that if an opponent discards the tile that will complete it, you can call ron and win**, and if you draw it, you call tsumo and win.
*there are two yaku that don't follow this structure, ignore them for now
**if you previously discarded a tile that could've completed your hand, you can't call ron
***there is a type of meld that happens when you have all four the same tile, ignore it for now
These are the basics of the basics, read more if it's too confusing
