What is the main development change that will occur during puberty?

Depends: If you are a girl the process is as follows: breast buds-->pubic hair-->growth spurt--> then menses. If you are a boy the process is not as straight forward: increased body size, penile and scrotum growth, pubic hair, and voice changes. The key is growth! the childlike body becomes more and more like an adult.

Rapid growth : Growth rate increases, secondary sexual characteristics like pubic hair, enlarging genitals, breast development, etc. Girls get periods, boys get facial hair and deeper voices. More lean body weight is gained from bone and muscle development. Emotional changes, etc. Lots if big changes happen during puberty.