As an example to show you step-by-step, we'll use r(adius)=12 cm. There is one formula to find the area.
1) A(rea of circle)=πr2 (Always write the formula your using as your first line)
2)A=3.14 x (122) (Write out the second line like so. It will usually specify whether or not you substitute π for 3.14, in this case it does. Put r2 into brackets, meaning in the next line, you will do this first)
3)A=3.14 x 144 ('2' means to square. Meaning you have to multiply the number by itself not by 2, lots of people get confused by this. Your third line should look like this)
4)A=452.2 cm2 (You then multiply 3.14 by your radius squared. There you have it! your answer! Don't forget to add what it's measured in plus squared)
Hope this helped you!