어제 저는 노숙자들을 돕는 자원봉사를 했습니다. - formal 어제 저는 노숙자들을 돕는 자원봉사를 했어요. - polite (less formal) 어제 나는 노숙자들을 돕는 자원봉사를 했어. - casual

Quick comment: I'm not sure if it was a typo, but it should be "thanks" instead of "thank's." As for your question, your first attempt was pretty close. Try this: "Yesterday, I volunteered to help homeless people." In this case, you are using "help" as a verb, which is why the sentence reads, "...to help...". This is in contrast to when "help" is used as a noun, such as when "asking for help." ~~~ A personal comment that is more for cultural insight: Some people in the United States, myself included, are now using the phrase, "people experiencing homelessness," to emphasize the human being experiencing homelessness, rather than labeling someone as "homeless." You see this cultural shift happening with mental health issues too, such as someone being referred to as, "a person with Schizophrenia," rather than, "a Schizophrenic." In short, it's a matter of personal choice and semantics. If you say, "homeless person," it's likely that no one will be offended. Most people won't think twice on it. But if you want to say, "person experiencing homelessness," that works too! The sentence would then read, "Yesterday, I volunteered to help people experiencing homelessness."