It depends on what the whole sentence is. If the subject of the sentence is singular, add 's'. If the subject is plural, don't add 's'. If the sentence is an imperative, don't add 's'.
"The boss believes me, guys." (singular subject)
"The girls believe me, guys." (plural subject)
"Believe me, guys." (imperative)
Just so you know, an imperative is the kind of sentence where it's like you are telling someone what to do. It starts with a simple verb (in this case, believe). Here are a few simple imperatives:
"Close the door."
"Open the window."
"Give me a pencil."
Hope this helps.
Student or Learner