#As for Sarai

The words "As for" introduce the next person God talks about.

#I will give you a son by her

"I will make her bear a son for you"

#she will become the mother of nations

"she will be the ancestor of many nations" (UDB) or "her descendants will become nations"

#Kings of peoples will come from her

"Kings of peoples will descend from her" or "Some of her descendants will be kings of peoples"