sometimes is an adverb just like never, always, or usually and you use it in the same way.
I sometimes sleep until 10 a.m., but I usually wake up earlier than that.
sometime = at a non-specific time
Do you want to see a movie with me sometime? (could mean tomorrow, Friday, next month, etc.)
I went to the museum sometime last year. (I only went once and I don't remember the date.)