Don't Look Behind You by Lois Duncan, published by Bantam Doubleday Dell Books for Young Readers, 1989.
Told in first person, this story definitely falls into the category of suspense rather than traditional mystery as April and the reader wait for the hitman to catch up with her family. Following the story of a teen forced into the witness protection program because of her father's testimony is a compelling setup. April (who later becomes Valerie) makes several mistakes that results in the hitman finding the family's home.
I found this was also made into a TV movie starring Patrick Duffy in 1999. Several of Duncan's other books have also been made into movies including Hotel for Dogs.