The most important thing is to caulk around any air gaps in the walls and ceiling. You need to seal around all penetrations at junction boxes for electrical devises etc. Think of it as if and air leak is the same as a sound leak. It also helps to use a clay or putty to wrap around the back of the junction boxes where the wires come in. Of course this is best done if the walls are open and the electricity of off.

Thank you very much!! i was wondering if you could tell me which kind of "foam" works the best ( the thing that i'm going to put in the walls.

If your walls are open, a standard R-11 fiberglass is considered sound deadening insulation. The foam product you are looking for is for gap sealing and small joint filling. Comes in spray cans, called Great Stuff. In doing recording studios or ceilings between apartments we have used a double drywall installation. A metal hanger called a hi-hat channel is screwed to joists after the first layer of drywall is installed. A second layer of drywall is screwed to the hi-hat metal. in extreme cases we build a second wall, or double stud wall so that there is no vibration possible between the two sides of the wall. In the most stringent install...the sound booth itself, we have installed lead sheeting. As you can see there are many strategies for sound deadening that can be tailored for your situation.

Ironwood's suggestions are all very good ones...