Common ratios are the ratio of one term in a series to the previous term. If you have 3,6,12...etc. your common ratio is going to be 2 because you're multiplying your previous term by 2 to get your next term. So the next 3 terms in my series would be 24, 48, 96. Each time I find a new number, it is the previous number multiplied by 2.
Common ratios aren't always 2, and sometimes they're even fractions, but you'll find that out later. Common ratios are contingent on the numbers you're given.
I hope I was able to help. Best of luck.