I would start with leg stretching as in yoga or ballet. This way you can build on your muscles. From here, if possible, try teaching yourself how to do the splits. It takes a great amount of patience, balance and flexibility to learn this, but it should help you when your standing (in your vertical state) to do an amazing high kick.
Best option is to train with MMA trainers, or try out some kick boxing. I train with MMA trainers and we do middle-high kick. Such as using power, speed and placement of foot when hitting punching bag and when leaving your foot off the floor.
Might sound stupid but if you want to kick higher, kick higher.
Stretch. Do some yoga.
I do muay thai and I have a powerful low kick and roundhouse kick but I really want to learn how to high kick with power.
