Hey!

Just take it slowly really. Stretch out regularly but don't overdo it at first. When I first started kf my kicks were rubbish, but once your legs get stronger and more flexible it will come. It's important to have a strong core area too so you can stay firmly planted and well balanced while kicking.
The best training is to practise as much as you can. Just do really basic drills ,work on your low round kicks first then get them really strong and start getting higher. No point in being able to kick high if there is no strength behind the kicks.