please don't.....sketch in your book for a long ass time before you start gettin shit up.  takes a long time to learn letterforms and colour schemes and honestly you are no where near being close to being viewed as "artistic" by the general public.
It's a good start but like what has been said before, pick a shadow direction and stick with it.Also mastering the letters on your keyboard (normal, straight up letters..) will help you a lot. start with that and proceed from there.
But please for the sake of graffiti, don't start painting walls yet.