Egads! You don't remember what an interjection is? It's a word or phrase that expresses surprise or some other emotion or is used as filler. An interjection often stands alone (gosh, darn). If an interjection is part of a sentence, it doesn't have a relation to other words in the sentence; if it's taken out of the sentence, the meaning is unchanged. Take a look at these sentences:

Hey, like, and ouch are interjections.

When you're expressing a strong emotion or surprise (as in Stop! or Darn it all!), use an exclamation point. If you're using milder emotion or merely using a filler (as in like or well), use a comma.

A note of caution about interjections: use them in moderation, if at all. In dialogue, interjections are used far more often than in more formal writing, where they're hardly ever used.