Here's how to get a P-value for a t-test in R (I always forget how to do this!) This uses the cumulative distribution function.
Cumulative distribution function for a t-distributed variable
If your t-value is 0.1558572 and degrees of freedom is 22:
It doesn't matter if your t-value is negative.
Cumultative distribution function for a Normally distributed variable
We can also calculate it for a Normally distributed variable, eg. P(Z <= 0.9), where Z is a standard Normal variable is:
Cumulative distribution function for a Poisson distributed variable
eg. P(X > 11), where X is a Poisson(8) variable:
> ppois(11, lower.tail=FALSE)