those whom he foreknew

"those whom he knew before he even created them"

he also predestined to be conformed

"he also planned in advance that they would be conformed"

"that he would change them to be like his Son"

This refers to Jesus, the Son of God. It is best to translate “Son” with the same word that your language uses to refer to a human son.

"so that his Son would be the firstborn"

"among many brothers and sisters who belong to the family of God"