What you need is a random number generator. Most software packages (or Excel) have an algorithm that will produce regular uniform or standard normal psuedo-random deviates.
For example, if you want to generate a standard exponential distribution (X) where the mean and standard deviation are both equal 1, then you would transform as follows:
X = -Ln(1-U)
where U is uniformally distributed i.e. U~[0,1].