This should be self-explanatory, but just in case it's not: your t-score goes in the T Score box, you stick your degrees of freedom in the DF box (N - 1 for single sample and dependent pairs, (N1 - 1) + (N2 - 1) for independent samples), select your significance level and whether you're testing a one or two-tailed hypothesis (if you're not sure, go with the defaults), then press the button.
If you need to derive a T Score from raw data, then you can find t test calculators here.
Significance Level:
One-tailed or two-tailed hypothesis?:
Enter your values above, then press "Calculate".
Additional T Statistic Calculators
If you're interested in using the t statistic for hypothesis testing and the like, then we have a number of other calculators that might help you.
T-Test Calculator for 2 Independent Means T-Test Calculator for 2 Dependent Means T-Test Calculator for a Single Sample