First put the numbers in order, least to greatest.
15, 17, 17, 18, 18, 19, 20, 21, 22, 23, 24, 25
Count the amount of number in the data set. In this one there is 12. Since 12 is an even number, the media will not be in the data set because there is no possible way to have the same amount of numbers on each side. SO, count 6 numbers in to the data set (which would be 19). Next to that is 20. Add 19+20=39. Divide 39 by 2 (2 numbers), which is 19.5
The median of this data set is 19.5
Hope I helped :)
