I am 18 years old and my breasts are really small. i don't even need a bra at all and i haven't started wearing one yet. i have 5 ft 6 inches height and my weight is around 45-46 kg. should i be concerned?

It's not the size: It's not the size that matters. A teen would have seen her pediatrician annually, and the doctor would have checked her breasts for proper development. A girl is ok if her breasts, pubic hair, and genitals matured properly (breast size is not what we look for but rather the changes in the nipples), especially if periods are regular. The doctor can re-examine and do some blood tests, if worried.

Ht, Wt, BMI: Your ht is at 75th %ile, wt at 4th %ile and your BMI is at 1st %ile. You are in other words considerably underweight. That may well affect your pubertal development. I don't know if you have had your first menses yet but you should examine what you eat and try to eat a richer diet than what you are presently do. You might get a peds endocrinology workup too.