To easily kill the guards in the Lighthouse level, after cutting the power, the guard will come to turn it on. Run past him as he comes through the doorway and he will say " Is someone there?". Run through the room. When you come back, you will find out that the lights are off and the guard is gone. He is around the corner. Equip your rifle and kill him.
To get the Thermal Suit costume in the Hokkaido level, find the wrench on the ground. Continue through the level until you reach the man who is shooting from the top of the building. After killing him, go to where he was. Then, run across the rooftop to the vent. Exit the room you fall into. The icon "Bash Door" should appear in the top left corner of the room. Bash the door and the costume will be in there.
To get the Displace Mercenary costume in the Display level, go to floor 3. To figure our which door has the costume, look through the optic cable. Once you see the green package, enter the room. The keypad lock code to the door is "5800".
To get the Shadownet Agent costume in Kokobu Soshu level, once you go down to the elevator shaft Sam will consider the Descent to Hell. There will be lasers at the bottom. Once you are down there find a ladder to get back up. Climb it, and you will end up in a small room with lasers. The costume is in the back corner.
To get the National Guard costume in Manhattan Streets level, go to the open area just before you enter the penthouse. Kill the two guards and climb the pipe at one of the ends of the area. Go left and jump down to the next platform. The costume is around the corner.
On the Displace level where you can kill two men, one resets all alarms while the other man sets them back to whatever number they were. The first man is in the room with the computer and books on the desk, near the boiler room. Knock him out first and shoot him with the 5-7 pistol. The next man is in the room after you get out of the vent shaft. You can kill any enemy on a no killing mission if you do it this way. The third person that does something on this level is the one guarding Milan's office. He leaves blood stains resembling a gun. Note: These only work if you wear the National Guard costume, which are found on the ledge in Manhattan. The ones featuring alarms (the first and second men) only work on a random basis.
To get the Argus Mercenary costume in the Machi Akko level, when you reach a hallway after falling out of a vent, there will be three room that require a lockpick to enter. In the third room will be a closet that cannot be opened. Continue to the bar, then go on stage and use the microphone. Sam will say something stupid. After he is done, turn around and look behind to see a keycard. This keycard will open the closet which contains the costume.
To get the Snow Guard costume in the Battery level, find the wrench in the bathroom. The costume is in the room that is shaped like a "U", and has two cameras and four lasers. Enter the keycard room in there. In the right corner is a guard standing in the dark. Kill him and climb on the crate. Then, turn towards the next crate and select the option "Bash Door". the costume will be in there.
To get the Masked Sam costume in the Penthouse level, go to Zherkezhi's apartment and enter the first room on the top floor. Kill the guard and go to the long dark hallway at the top left of the room. There is a keypad lock to get in. The keypad code is "5698".
To get the short sleeved Sam costume in the Bank level, when you must steal millions of dollars by using the lockpick, go to the small vault left of the computer. Use the lockpick, and the costume will drop out.
Tom Clancy's Splinter Cell: Chaos Theory Nintendo DS Q&A
Stupid question but how do I use a gun?
How do i use a gun
Answer from RaiderOfTombs
There's a box in the lower left hand corner of the touch screen that shows what item you are using. Make sure you tap that and change your item to a gun.To draw your weapon, press R. To fire, press L. To holster, press R again while your weapon is drawn.