For scoring, I highly suggest you read the
wikipedia entry.
As for dora, dora is an additional bonus multiplier that counts for extra yaku/fan/han. (Dora does not count toward the 1 yaku/fan/han minimum.) If it is one of the non-honors tile, it is the next sequential tile from the dora indicator tile on the wall.
Ex: Dora indicator is 5 of circles, the 6 of circles is the dora tile.
If the dora indicator is the 9 of any set, the dora is the 1 of the same set.
Ex: The indicator tile is the 9 of characters, the 1 of characters is the dora tile.
If the indicator is one of the dragon tiles, then it is the dragon tile of the next color:
Red->White->Green->Red
If it is a wind tile, then it is the next sequential wind:
E->S->W->N->E
Reverse dora is the same except you only reveal the indicator tile after winning when declaring riichi.