You can improve the mechanics of your kicks by practicing it slowly, making sure to focus on each of the four main parts to a kick: Chamber, strike, rechamber, plant.
This is very important. First get the mechanics down, then add speed and strength.
We have beginners gradually work on the heigth of their kicks. Knee heigth > belt heigth > chest heigth and maybe eventually head heigth, but the latter isn't necessary.
Also make shure to stretch out the foot so your instep is in line with your shin and pull those toes up.