As an example to show you step-by-step, we'll use the number r(adius)=4.6 cm. I'll show you how to find the circumference if you have the radius.
1) C(ircumference)=2πr (This is the formula we use to find the circumference, always write this down first when beginning)
2) C=2 x π x 4.6 (Write out the second line like so but, substitute r for whatever your radius is)
3) C=6.28 x 4.6 (Multiply 2 x π (in most cases it will say to use 3.14 for π), you should get the answer of 6.28. Write that in the place of 2 x π like so)
4) C=28.8 cm (Multiply 6.28 x whatever your radius is and there you have it! the circumference. It will specify if you should round your answer or not, I've rounded the answer to the nearest tenth. Don't forget to include what it's measured in if it said)
I hope this helped you!