# 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"

# to be conformed to the image of his Son

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

# 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.

# that he might be the firstborn

"so that his Son would be the firstborn"

# among many brothers

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