Build up your technique before you work on your endurance.
Being tense is fine... you want a body tone of 7/10, with 10 being like an "army man" and 1 being sloppy. With more swimming you'll get better with this.
Work on rotating. Do 6-10 kicks on one side, then rotate to the other hip. So say, start on your left side, kick (initiate from the core), do three strokes, then kick on the other side.
Remember to "snap" your hips from side to side.