ok when i play middle C the tone is not full (if that makes any sense, i don`t know how else to describe it) how can i solve it.

check the pad, is it damaged? or offset

You mean it overtones? Yup, happens to me... it`s to do with your embouchure - too tight.

You should adjust your thoat and for an exercise see if you can get air on the top of your mouth when you play, this helps with tight embouchure. It`s just a thing you have to learn, I know, it used to happen to me all the time. Also, breathe from your diaphram - it works wonders.

is overtone when two notes are mixed toghether or somthing

Overtone is when u play a note with higher notes also sounding at the same time. All notes on accoustic instruments have naturally occuring overtones, but they are usually really soft, so that you only hear the primary tone. If you play a low note, like low B, and you set up ur embouchure wrong, you can get some overtones.

a overtone is like multiphonics?

Do you have a recorder?
Try playing low C on it. That`s the way you should blow.