Hip rotation is very important with freestyle swimming. When it is in sync with shoulder rotation, it ensures a long stroke that will maximize your efficiency. Initiate the rotation from your hips and this will help your core and lats power the stroke.
Try a couple of drills that can help you get started. The 636 drill (my favorite) starts by pushing off the wall on one side with the bottom arm out stretched in front of your head. The other arm is resting on top of the hip along the leg. Perform 6 kicks on your side and then take 3 strokes to get to the other side. I prefer to take a breath during my 3 strokes and then hold my breath during the 6 kicks. Another quick drill to help you add awareness to your hip rotation is kick board pulling. Place a kick board vertically between your upper legs in place of a pull buoy. Begin swimming using good hip rotation and try to feel the top end of the kick board touch the water at full rotation. Incorporate these drills into your warm up and cool down and soon you'll be swimming more efficiently and faster!!
