Probability and t-distribution

[Dogs]

Disclaimer: this isn't actually poker related and hence should really belong in off-topic, but I figured more maths-folk would see it here. If it needs to be moved, please do.

I'm working on a piece of software where I need to replicate the Excel function TDIST; an explanation of this function from the help is:

Returns the Percentage Points (probability) for the Student t-distribution where a numeric value (x) is a calculated value of t for which the Percentage Points are to be computed. The t-distribution is used in the hypothesis testing of small sample data sets. Use this function in place of a table of critical values for the t-distribution.

Syntax

TDIST(x,degrees_freedom,tails)
X is the numeric value at which to evaluate the distribution.
Degrees_freedom is an integer indicating the number of degrees of freedom.
Tails specifies the number of distribution tails to return. If tails = 1, TDIST returns the one-tailed distribution. If tails = 2, TDIST returns the two-tailed distribution.

I have all the values to input (it's one-tailed incidentally), but don't know how to arrive at the result. Is there anyone who can walk me through the maths behind it?