To find the mean you have to get the average number found by adding all data points and dividing by the number of data points.
Example: The mean of 4, 1, and 7 is (4+1+7)/3 = 12/3 = 4
For median you have to get the middle number which is found by ordering all data points and picking out the one in the middle (or if there are two middle numbers, taking the mean of those two numbers).
Example: The median of 4, 1, and 7 is 4
because when the numbers are put in order (1, 4, 7) the number 4 is in the middle.
To get the mode you have to get the most frequent number which is, the number that occurs the highest number of times.
Example: The mode of {4,2,4,3,2,2} is 2 because it occurs three times, which is more than any other number.
Hope that helps.