yeah if going to paint, the type of paint matters...some types are best for outside, some for bathroom, some for bedrooms, slightly different finish to them. pink and white would be best, so looks good no matter how many other hello kitty things you add. if theres no one to help, you could talk to the person sellin the paint, they should be able to recomend paint types etc. oh and need drop cloth so you floor doesnt end up painted too :p
if painting is not an option, then...like above said, curtains etc, maybe a poster, somethin for your bed...to save some money if want/need, could go for shade of pink sheets close to hello kitty shade with white comforter/duvet whatever you use and add hello kitty pillows. much cheaper than gettin a whole hello kitty sheet set and depending on bed size might not be an option anyways. companies seem to think only people with twin size beds like character bedding
before makin any choices look around at whats out there, im sure theres lots! online too. what the walls look like is prob the first decision should make as affects the whole room and whatd look good with the walls.