Add the -s only when the imperative is followed by a pronoun that starts with a vowel (which means y or en). Add -s only when the pronoun is a complement of the verb, not when it’s the beginning of a longer expression that happens to follow the verb. There’s always a hyphen after the s.

Vas-y.
Va en classe.   (en)
Va en parler avec lui.   (en is a complement of parler)
Vas-y (pour) voir ce qui s’y passe.   (“Go now and see what is happening there” — y is a complement of voir)
Va y voir ce qui s’y passe.   (“Go there to see what is happening there” — y is a complement of voir)

For natives, the rule is “add an -s if it there’s a [z] sound”, which obviously only helps if you speak French fluently and are learning to spell it. Even so, natives sometimes get this wrong. There’s no s at the end of the imperative singular for first group (-er infinitive) verbs because it’s derived from a Latin ending which was just a (long) vowel, but there’s an s at the end of the corresponding indicative form which derives from a Latin -s ending. I suspect that in a few centuries, the imperative singular will align with the indicative (this has already happened with -ir and -re verbs). In the meantime, “no -s except before complement pronouns that start with a vowel” is a rule that natives do mostly follow in spoken language, even if they’d occasionally misspell it.