Kickboxing/Muay Thai

Make sure you have good form when you kick even if your foot is not very high off the ground but you probably want to try to go higher is you can.

I would note that a good kick doesn't necessarily mean a "high" kick. There are certain styles that would emphasize kicks to the abdomen or lower, with punches and elbows as upper body attacks, because of the time and distance it takes to pull off. In sparring I tend to use more side or push kicks for distance and round kicks to the legs, not bothering much with high kicks... so just because kicks aren't to the head don't mean they aren't good.

As mentioned stretching a lot helps, and of course core and leg strength. You can do a lot with body weight exercises like squats and bag work if you have access.

I would say stretching is good. I am also a yoga practitioner. It also helps to have a good instructor to make sure you are kicking correctly. As a Muay Thai practitioner, I would say make sure you turn over your hip. One of my instructors used to say we need to remember to show our butt to the bag that we are kicking by the time we're done.

Hello,
lately i've been trying some kicking exercises but very hard, i can hardly lift my leg high enough to kick.
Any tips and exercises? or is it just a matter of time before my shots get better?

You may want to check out flexibility.maestro on youtube or insta. He has some great stretching and workout suggestions to improve kicks. I trained with him for a bit and he's really wonderful.

I suggest you Tom Kurz's "Power high kicks with no warm-up".

If you have trouble lifting your legs high enough, then most likely your core is weak.