To find the median of a set of number you must first arrange them in order starting with the smallest and going to the largest. So,
54, 55, 55, 55, 56, 58, 58, 61
Now the median is just the middle number, but since we have an even amount of numbers there is no real middle. So the median will be the number in between the middle 2 numbers. So the middle 2 numbers are 55 and 56 and the number in between those 2 numbers is 55.5
So 55.5 is the median of this set of numbers
Feel free to ask any questions if you are still confused.