i think the first question to answer is, why do you feel the need to cut?
if you do it for the pain, it doesn't really matter where you cut. if you don't want to end up in hospital or dead, you shouldn't cut too deeply and you should probably avoid your inside wrists just in case. if you want people to see it, use your forearms. if you want to hide it, use your upper arms or legs. as far as i know there aren't any rules.
but if you are only doing it for attention - and don't take this the wrong way, but that is the impression your post gives - maybe you should think about getting some help. i only cut myself when i absolutely have to feel something else other than what's in my head. doing it so that people will think you are interestingly messed-up will only work for so long before people get tired of it.
but most importantly of all - PLEASE DON'T KILL YOURSELF. it's so wasteful and think about what it will do to people. things can and will get better. think about someone or something you love. if you kill yourself you will never have anything again.