How can I lower the strings to make it closer to the fretboard? When I play my guitar, using the 5th fret until to the 12th fret, it seems the sound doesn't come to my ears gently and I thought it is because of the level of the strings.
Not 100% certain what you mean by 'doesn't come to my ears gently' but lowering the strings can be very simple or tricky depending on what type of guitar you have.
A final thought: have you compared with other guitars and confirmed that the issue is string height? Maybe you have a thicker gauge of string than would suit you...
You don't tell us if you have an electric or an acoustic guitar. If electric, there are standards set-up measurements for most models. The bridges on most models are highly adjustable, so it's mostly a matter of getting a decent reference page and a good precision ruler and following the instructions.
On an acoustic, it's a bit more complex. There are two action adjustments; the nut and the saddle. Again, there are more-or-less standard measurements as far as the basic set-up goes, but these are dependent on how you play. Someone with a light fingerstyle approach can use a lower action that a strong flatpicker who really digs in. Anyway, I recommend going to the excellent Frets.com site and following the basic adjustment sections: http://frets.com/FRETSPages/pagelist.html#Musician
This will give you the ball-park measurements and you can then decide if you want to try to do it yourself or take it to a shop.