Count up how many of each score there are. For example, if you were measuring scores on a 10-point quiz from your class, you would count up how many people scored 0, how many scored 1, all the way up to 10.
Select the score with the highest frequency (most people in that category). If there is more than one tied for top, select them all.
Write "The mode is..." or "The modes are..." and then write the score(s) that got the highest frequency.