Complex sentence: Josh once had a girlfriend, Mindy Crenshaw-however, the two did not always get along they hated each other; starting at 12 years old, the two often battled against each other in their school's annual Science Fair.
Split into simpler sentences: Josh once had a girlfriend, Mindy Crenshaw-however, the two did not always get along. In fact, they hated each other; starting at 12 years old, the two often battled against each other in their school's annual Science Fair.