Well before you do either it would be better if you gave more information, you don't explain anything when you ask like that or maybe you'd like us to guess first, no just kidding, You need to describe more info as to what is wrong and if you could post some photo's would help also.Ship
Ship is right. You haven't given enough info on the problem. What is happening? Is the action too high (hard to play) or too low (buzzing noises)?
What brand/model of guitar is it?
Also, your profile states you live in Milano, Italy. What is the weather like this time of year? How cold does it get? The issue you are experiencing could very well be related to humidity. Is the guitar always sitting out? Get something to check the humidity in the room and in the guitar. If the air is dry then put the guitar in its case for a few days and get something to help humidify the guitar. I have Damp-Its in all my acoustic guitars and they work very well.
Hopefully this info will give a little insight but giving more info about the problem will help.
My problem is i have changed the saddle of my 12strings but i do not know if i have to lower it or adjust the trussrod.Thank you John for your involvement and scuse me for my english language.Hello,Ray
There should be no need to adjust the truss rod if there was no problem with playing the guitar before changing the saddle.
Do you still have the old saddle? New saddles are oversized and normally have to be sized to the instrument. Great care has to be taken in doing things like making the new saddle shorter. If you still have the old saddle use it as a reference. You'll probably notice the new saddle is higher than the old. You can try sanding the bottom of the new saddle down to the point that it's close to the old saddle's height. You have to be careful to sand the bottom perfectly flat and at 90 degrees to the sides.
Your best bet would be to take the guitar to a local luthier to have the work done. The luthier will be able to check out the whole guitar as well and do a setup that is appropriate for the strings and your style of playing.
Humidity: Since there is high humidity in Milano you should keep the guitar in its case for the most part. Or if you do keep the guitar out, monitor the humidty in the room. The humidity should be between 40% - 47%. Too much and the guitar will swell which can cause bad things to happen.
I should speak Italian with you, Ray(?), because I'm Italian too, but it wouldn't be fair with the others here. You can send me a message at email@example.com if you want. I'm a little south of Ancona.
You say you changed the saddle. Why? Which material? Who did it, yourself or an expert? You didn't answer the most important point: which is the problem? I presume the strings are too high, if you say you would lower the saddle.
How was the situation before you changed the saddle? Is the nut well shaped?
As I do many guitar settings, let me know in details (and in Italian, if it's easier for you).