Semicolon, it goes in between words, not sentences. Period is the most common one to go behind a sentence, then a quiestion mark for questions, and exclamation for excitement.
Examples:
"Dawn's room needs to be cleaned; it is very messy and unorganized."
"Dawn, make sure you clean up your room."
"Dawn, did you clean up your room today?"
"Dawn, clean up your room!"
