To hold your guitar correctly, you must make a "clip" between the thumb (behind the neck) and the top of your palm. First of all, practice your left hand. To start, put your hand on the 5th fret.

The hand position is very important, here are some tips:

1. All fingers except the thumb are on the strings.

2. The thumb is opposite to the major.

3. Relax. Your hand has to move quickly.

4.Each finger is independent.

You can now try to play a note: La, for example. Place your index at the lowest string (Mi at the top). Press hard. Your fingers have to be rounded. Now play the string with your right hand. It will sizzle. This is normal, you have to press hard enough and not be too close to the hoop. Practice that a lot.