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" 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"