does somebody know how to play the glissando exactly?

My teacher said that I have to use my left-side of my thumb. But it's hurt if I use it.

My friend said that I have to use my back-side hand. Sometimes using nail is ok. Which one is right?

Thanx for your help

Both are correct. So is using the underside of your finger tips. The point is to learn and use techniques that produce the gliss acurately every time.

What is a glissando?

You can watch House M.D. and a savant perform a gliss(ando) here:

http://tinyurl.com/32fwrz

Standard practice is to use the nails of your fingers when playing an ascending glissando and the nail of your thumb when playing a descending glissando. If your teacher advised you differently, it may have to do with the peculiarity of the particular passage in question.

What is a glissando?

The term means different things on different instruments. On the piano it means playing either the white keys or the black keys in succession rapidly without articulating the notes individually. On the guitar it means sliding your finger up or down a string. On violin-family instruments it also means sliding your finger up or down a string, but the effect will be different because violin-family instruments have no frets; the effect on these instruments is called portamento, an effect impossible to achieve on the piano. To achieve a portamento effect on the guitar you bend one of its strings.