Mean is the average so just add up all the numbers in your data set and divide by the total for numbers that there are.
Ex. (5, 7, 8, 9)
5+7+8+9=29
29/4=7.25
7.25 is the Mean.
Median is the number that is directly in the middle so just count inwards from each side after you ordered your data set. If you get two numbers just add them both and divide by two.