There are already quite a number of articles related to taatsu theory. If you're purely going for the fastest way to reach tenpai, you should start discarding in order, beginning with the taatsu with the lowest possibility of effective tiles.
If you find yourself with similar taatsu, you can consider with the following:
a) future evolution of the taatsu (penchan good evolution etc)
b) is it a bad shape, for example having effective tiles overlapping
c) how easy it is to win when in tenpai
d) yaku etc
Among them, a and b are the most important.
This is choosing between discarding 89 pin or 12 sou. The readers who have read the article about overlapping effective tiles , should be able to easily find the answer.
As 89's effective tiles overlap with 68, discarding 9 pin is the correct answer.
This problem can be attempted with the "loss of effective tiles" method, the correct answer would be the discard with the lowest loss of effective tiles.
Discarding 12 sou: the loss of effective tiles include 3 sou along with 1/2 sou
While discarding 9 pin, the loss will only be 9 pin, therefore discarding 9 pin is better than 12 sou.
This is a rather famous beginner tile efficiency problem(smile): Choosing between 23 wan, 78 wan, 34 pin.
The correct answer is discarding 8 wan. After you discard 8 wan, you're left with the shape 233457, 6 wan is still an effective tile! If you discard the remaining two taatsu, you'll lose two types of effective tiles.
Like the 233457 shape, it's very common in Japanese mahjong, beginners should pay more attention.
This question is harder.
68 wan and 24 pin are both kanchan in the same position, both seem to be similar.
But if you consider the evolution: when 68 wan draw 5 wan, 2356 wan becomes the aforementioned overlap of effective tiles shape. Therefore the value of 68 wan is lower, you should discard it first.