The package rand can be used to generate pseudo random numbers, which are generated based on a specific initial value (called "seed").
A popular choice for this initial seed is for example the current time in nanoseconds - a value which will probably differ when you execute your program multiple times. You can initialize the random generator with the current time with something like this:
rand.Seed(time.Now().UnixNano())
(don't forget to import the time package for that)
There is also another package called crypto/rand which can be used to generate better random values (this generator might also take the user's mouse movements, the current heat of the processor and a lot of other factors into account). However, the functions in this package are several times slower and, unless you don't write a pass-phrase generator (or other security related stuff), the normal rand package is probably fine.