The correct phrase depends on the context of the sentence.
When rooting for a team, the correct phrase is “let’s go.” “Let’s” is a contraction of “let us.” Saying “let’s go, team” means you’re cheering them on as a fan or as part of the team. “Let’s go” can also be used to express impatience with someone (e.g., “Let’s go! We don’t have all day”) or, in a more literal sense, to suggest that you and at least one other person depart (e.g., “Let’s go home”).
“Lets go” only makes sense when you’re using “lets” as the third-person singular present tense form of the verb phrase “let go,” meaning “release” (e.g., “She always lets go of the rope too soon”).
The QuillBot Paraphraser can help you rewrite sentences to express your meaning clearly.