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.
- If you have adjustable bridge pieces then the answer is - adjust them. Very easy.
- If you have a floating bridge with fixed bridge pieces you may be able to raise or lower the entire bridge by adjusting the two screws it pivots around. As @ Markus says, it is important to slacken the strings while you do this to stop the edges of the screws from getting chewed up.
- It is possible, and tricky to tell from your description, that you may actually need to adjust the truss rod, which alters the curve of the entire neck - this is a much more sensitive job which can cause major issues if you get it wrong, so I would advise leaving this to a luthier if you don't know what you are doing.
- Another possibility is that some of the frets may be worn more than others, which can make the string height vary along the neck - fret replacement is another job best left to experienced luthiers.
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.
Now, as to that truss rod... The truss rod is not normally used for action adjustments, though many guitarists seem to think the opposite. The truss rod is for setting neck relief to avoid string buzzing. Adjustment may affect the action, but that is not the purpose. Once neck relief is properly set, then the action is adjusted.
Beyond what Dr. Mayhem suggests, there could be a minimum of how low your (electric guitar) bridge can go, and the solution there is to add a shim in the neck pocket on the bridge side. This changes the neck angle, which has a slight change at the neck but a great change at the bridge. It allowed me to get the action of my Telecaster very low.
Of course, this only works if your guitar has a bolt-on neck.
You really don't want to adjust this by yourself if you are not a technician. There is a metal rod that runs across the neck, look through the sound hole and you will find it. You can adjust this with an allen wrench to curve the neck and so lower or raise the strings.
Be careful here, because if you go too far you might ruin the neck. The technician can also carefully shave the the little white piece that you can see below the pins. Again, this piece can often be made from bone and could be very expensive, so don't risk messing it up.