This should be self-explanatory, but just in case it's not: your F-ratio value goes in the F-ratio value box, you stick your degrees of freedom for the numerator (between-treatments) in the DF - numerator box, your degrees of freedom for the denominator (within-treatments) in the DF - denominator box, select your significance level, then press the "Calculate" button.
If you need to derive an f-ratio value from raw data, you can find an ANOVA calculator here.
Significance Level:
Enter your values above, then press "Calculate".
ANOVA Calculators
This site features a couple of different ANOVA calculators, and also their non-parametric equivalents, that you might find useful.
One-Way ANOVA Calculator for Independent Measures One-Way ANOVA Calculator for Repeated Measures Kruskal-Wallis Test Calculator for Independent Measures The Friedman Test for Repeated Measures