Sometime is a vague and unspecified time at which you do something. It's use would be the same as saying, "When I get around to it." I like how Tristi explains "some time" so I'll just quote from her writing tip: "The words "some time" mean, "I need a quantity of time." "Some" is used to refer to a quantity." So, it is like needing some salt or some proof. You are asking for an unspecified quantity of time.
Also, if you can put a word like "spare" in the middle of it, then "some time" should be the option to go with.
When I have some time I need to start outlining my story.
I need to update my blog somesparetime.
When I have some spare time I need to start outlining my story.