I am not sure how high you actually want to kick, but each style has a certain archetypal style of kick they follow. Basically, that a high kick in one style can be different from another with slightly different mechanics.
A high round house kick, however requires both muscle strength and flexibility. The muscle strength will be in the quads, hamstrings, and the glute.
The flexibility will be in and around the hip joint as well as the leg itself...in particular, the supporting leg of the kick.
Then, high kicks require that requisite practice to be able to move past that staccato movement where you are breaking down the motions of the kick into its constituent parts and smoothing it out from chamber, use of the hip and finally throwing the kicking leg out there.
Practice....and the some more practice...and you'll get it.