Using that, which and who to introduce clauses and when to use commas:
The little boy who lives next door is a noisy, hell-raising brat.
"Who lives next store" describes one little boy in particular, not the entire neighborhood of nice and respectful children; therefore, that clause is not set off by commas. Those four highlighted words comprise a restrictive clause.
My brother, who lives in Tibet, is not a spiritual man.
"Who lives in Tibet" is not essential to the sentence. Bro could live in Brooklyn Heights and still not be a spiritual man.
Who is used when people are the subject.
Mother's portrait, which had been hanging over the fireplace mantle, is in need of restoration because of faulty fluing.
"which had been hanging over the fireplace mantle" is helpful but nonessential information. Use which to introduce a non-restrictive clause and set it off with commas.
The book that was stolen from the library was a first edition.
"that was stolen from the library" is a restrictive clause, introduced by that, and is not set off by commas. It is one book amongst thousands that was pilfered.