Sit or stand up straight. Think of your back making a straight line.
When playing the flute, stand with your feet planted firmly on the ground. Make sure your feet are in line with your knees and hips. Bend kness slightly to avoid locking knee joints.
Keep your neck and shoulders relaxed while playing the flute. Do some head rolls to loosen up before you play. Keep your shoulders back and down. Do some shoulder rolls also!
Use your stomach to breathe. An exercise to practice this is: Lie on your back, put your hand on your stomach. See how your stomach rises and falls with your breathing. Now stand up and put your hand on your stomach and push the stomach out when you breathe in. Use your stomach muscles to push the air out so your stomach should go in when you breathe out.
When practicing your scales, start back at the beginning if you make a mistake. It's important to play the whole scale through perfectly from the first note to the last. Don't just go back to where you made the mistake, go all the way back to the beginning of that scale.