Is it true that ovulation occurs before the very first period ever for a young girl? i have read some books, couple state that it's quite rare.

It can: Ovulation is somewhat irregular in the first several months after menarche and may remain so for a year. However, it can occur as the body prepares for menarche, leading to pregnancy (if exposed) prior to menarche. My first delivery in training was a 13 yo with this history.

Strict definition: In reproductive age females: both women and girls, menstruation (a period) is always by definition caused by a drop in Progesterone after ovulation. Thus any spontaneous (meaning no medicines taken) menstrual period at any age is always the result of an ovulation. It gets confused because there can be vaginal bleeding/spotting for other reasons, people may call "a period" - but it is not.