There are 3 different meanings of this word. Although this seems confusing at first the differences are nice and easy to remember!
Some time - this means a period or length of time. We don't know how long it is, but we know it's more than nothing but less than forever.
Sometimes - this means occasionally or every now and then. It is an adverb of frequency. Think of it as halfway between never and always.
Sometime - this means at some point in time, when exactly is not known or not given.