To compute the probability function, follow these steps:

1. Identify the random variable for which you want to compute the probability function.
2. Determine the possible values that the random variable can take on.
3. For each possible value, calculate the probability that the random variable takes on that value.
4. Write down the probability function, which is a function that assigns a probability to each possible value of the random variable.

For example, suppose you want to compute the probability function for the number of heads that come up when flipping a fair coin three times. The random variable is the number of heads, which can take on the values 0, 1, 2, or 3. The probability of getting 0 heads is 1/8, the probability of getting 1 head is 3/8, the probability of getting 2 heads is 3/8, and the probability of getting 3 heads is 1/8. Therefore, the probability function is:

P(X=0) = 1/8
P(X=1) = 3/8
P(X=2) = 3/8
P(X=3) = 1/8

Note that the probabilities must add up to 1.