Percent means "per one hundred" so it is value/possible then * 100. So if you have 5 apples out of a possible 7, that's 5/7*100 = 71.4%
In C2 you can use the 'round' expression to round to the nearest integer (whole number). so round(5/7*100) would give you 71. If you want it to have the % symbol, just use &
On Action---Set Text to round(5/7*100) & "%"
Will set the text of a text object to 71%.
Also, How Do I is generally (to me) for more in-depth questions whereas Beginnger's Questions is simpler questions for people starting out with C2.