toggle- i am going to answer your questions up front.
1. where to cut- anywhere. cutting is whatever you want it to be. you can cut anywhere and cut anything. but dont go for depth. shallow cuts can release as much anger as a deep cut, and deep cuts will bleed, and bleed and bleed. the clean-up is hard.
2. how to hide/excuses- first, long sleeved shirts are a life-saver! if you have cuts on your arm, wear long sleeves, a sweat-shirt maybe. i where a sweat-shirt until my cuts have healed. when they heal i cover them with make-up. you, are a boy so that may be hard for you to get make-up. but it covers well, usually. try using your mothers when she is not around. take her foundation and put a dab over the scar and blend it. the scar's color will become dull and less red. put on as much as you need to hide it. you do not have to cover it completetly, if some color show through, it will hardly be noticed. also try under-eye make-up for womens dark circles they get under their eyes. since it is so light colored, it covers reallly well, better than foundation. apply same as other. if someone does see them, say it is a cat scratch, a dog, a bird, of yours or say a friend's pet, cat etc, scratched you. or if its alittle bigger deeper cut, say you were cut on a metal fence, you fell or something.
i hope that my little advice helps, take care.
ps. the picture, with your name, if its you, your cute.