Since this same question is asked in the Bhagavad-gita (8.1),
I would suggest that you read about it there. The best translation
that I am aware of is the Bhagavad-Gita As It Is, by His Divine
Grace A.C. Bhaktivedanta Swami Prabhupada. It's probably in your
school or local public library, since it has been acclaimed by
leading scholars. The commentary is also very clear and easy to
read. You will find the concept of "Brahman" mentioned throughout
the book too. Good luck!