The dependent variable generally changes value in response to a change in the independent variable. In some situations you can treat either variable as dependent/independent. Often you can think of which variable do you control.
(1) rainfall;crop yield In general the more rain the better the crop yield. (Except in cases of flooding, or inability to plant crops at the beginning of the season, etc...) However, better crop yields do not "cause" more rain -- the rain either comes or not, independent of the crop yield. The crop yield depends on the rain.
Rain - independent Crop yield - dependent
(2) Taxi fee; length of ride Again the amount of the fee increases as the length of the ride increases.
Taxi fee - dependent Length of ride - independent
Rainfall is x, crop yield is y
Length of ride is x, taxi fee is y