This should be self-explanatory, but just in case it's not: your r score goes in
the R Score box, the number of pairs in your sample goes in the N box (you
must have at least 3 pairs), then you select your significance level and press the
Enter your values for Pearson (r) and the number of pairs in your sample, and then press the button.
This site features a number of different correlation calculators which you might find helpful.