I need help to get my roundhouse to the head and I need to improve my technique

It is hard to critique your technique without looking at it, but I will say this: a big factor in roundhouses to the head is flexibility.

Maybe training with bag will hepl you. Flexibility is very important, but without technique and power your roundhouse will doesn't have any sens.

I agree. You have to have the flexibility. Here's a good thread on that:

 

http://www.karateforums.com/viewtopic.php?t=6186

 

But technique is more important. I seen people have great flexibility, but because their technique wasn't there, they couldn't kick to the head. And yes, you also have to develope power. Working on the heavy bag will help with that.

totally. you need flexibility. i have pretty good flexibility but i am not totally comfortable kicking that high so i practice it in class. that also might help you. and i agree with the bag work. it should help!

this is great but can you help my with speed

Sure! Practice, practice, practice. Start slowly, get it right, then build up speed.

practise

practise

exsactly what i was gonna say!

Speed comes from developing the muscles involved in the kick by repetition and dynamic stretching. Flexibility of the spine, hips, knees, and ankles are also important to the technical elements of the kick. When it all comes together-wham!