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, (N_{1} - 1) + (N_{2} - 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.
Want to know how to report the result of a t-test (APA style)?
Significance Level: .01.05.10
One-tailed or two-tailed hypothesis?: One-tailedTwo-tailed
Enter your values for T Score and degrees of freedom, and then press the button.
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