The tip percentage is multiplied by the amount you spent. Some calculators will do that directly. More commonly, you need to convert the percentage to a decimal value and then do the multiplication.
Many people get hung up on percentages and what they mean. The percent symbol (%) is just a fancy way to show "divided by 100". (% means /100.)
Your quantity 15% is 15/100 (fifteen hundredths) or 0.15 (fifteen hundredths).
So, 15% of 24 is (15/100)×24 = 3.60. This is the amount of a 15% tip on $24.00.
_____
15% is a fairly easy number to work with when computing tips. It is 10% + 5%, which is to say, it is 10% and half of 10%, added together.
Of course, 10% is 10/100 = 1/10. You know you can multiply by 1/10 simply by moving the decimal point. So 10% of 24.00 is 2.40. It is easy to compute half that amount in your head, as all you need to do is divide the digits by 2 to get
... 2.40 ÷ 2 = 1.20
Now you know the amount of 10% (2.40) and 5% (1.20) and you can add them together to get 15%:
... 15% of 24.00 = 2.40 + 1.20 = 3.60
_____
20% is another easy number to use for tips, as it is 10% × 2.
Likewise, 18% is not too difficult, as it is
... 18% = 20% - 2%
... = 20% - (1/10) × 20%.
Then 18% of $24.00 = $4.80 -0.48 = $4.32