P-value is the probability to get the current statistic result under the assumption that H0 is correct.
If you decide to reject the H0, P-value is the probability of type I error - rejecting a correct H0. A commonly used rule defines a significance level of 0.05. When the p-value is smaller than the significance level, you can reject the null hypothesis with a little chance of error. Another commonly used significance level is 0.01.
If you know the statistic value, choose the relevant distribution otherwise use one for the above test buttons.