"Sometime" generally refers to an uncertain future time. For example: "I would like to meet your sister sometime." This means that you want to meet the person's sister in the future, but you're not sure when that would happen. "Sometimes" is generally used to describe things that happen occasionally. For example: "I go to the theatre sometimes." "Some time" isn't used as often. It is usually used to describe an ongoing situation that has been happening for a long time. For example: "I have been struggling with mathematics for some time." This could mean that you've been struggling with math for years. This might help: http://www.grammarly.com/handbook/grammar/adjectives-and-adverbs/19/sometime-sometimes-and-some-time/

Just one thing to add to Angela's very thorough answer: You never actually need to use the adverb 'sometime', as in the first example. I never use it. You can write it as two separate words, 'some' and 'time', and consider it to be two separate ideas. I hope to go there again some day. We must have dinner together some evening. I would like to meet your sister some time. 'Some' simply means 'one unspecified xx', where 'xx' is a period of time - some afternoon, some morning, some time. If it makes it less confusing for you, you can think of, and write, 'some' and 'time' as separate words. This will make it easier for you not to mix it up with the adverb 'sometimes', which means 'occasionally'.