Hi , i saw many plp doing a side kick in the wushufixed and instagibfeet , etc. i wanna learn it plz can some1 tell me what joints to contract , extend etc.?

i saw them almost hitting the ground in first ground then with the right leg side kicking me

The move is called sweep kick.
The idea is to lower your shoulder and fall down so you are parallel to the ground. When you are close to the ground you raise the shoulder and push it against the ground to launch yourself forward in a spinny motion.

A quite simple one is
Relax all
Extend left hip
Extend right glute
Right bending lumbar
Right rotating chest
Extend right pecs
Lower right shoulder
Hold right elbow
Hold right wrist
After 50 frames you can launch yourself. Just adjust some joints to aim it.

what i tried against uke , it just makes a weak kick , besides when it finishes i cant get up and lose....

thats because you need to practice to make your kick stronger, and if you cant get up than you REALLY need practice.
To be honest whenever I did it, I could only do a backheel

i gotta do all that without or with C??btw how can i make my kick stronger

without, if you do press C, you will just stiffen up and most likely fall

To make your kick stronger, move every one of your joints into it. You can relax or hold unneeded joints. (tip: relax looks cooler, and in most cases is the better option) rely on the ghost in SP, but not too much. Press b.

No joints are un-needed, don't old them.
Some things like your lumbar or Ankles cannot be held, or your ragdoll will stiffen up a bit. Keep your neck relaxed though, Unless when you see a punch coming, you can use it against them.

It's not that hard.

and hampa kick its easy? how to do it??