We use who when referring to people. We use which or that when referring to animals or things. Note that that can be used in place of whom when referring to people.

The girl won the first prize. She was congratulated by all.
The girl who won the first prize was congratulated by all.

John is a clever boy. He has been chosen for the scholarship.
John, who is a clever boy, has been chosen for the scholarship.
OR John who has been chosen for the scholarship is a clever boy.

Mrs Simon won the championship. She practices everyday.
Mrs Simon who practices everyday won the championship.
Mrs Simon who won the tournament practices everyday.

The jet was grounded. It has engine trouble.
The jet which had engine trouble was grounded.
The jet which was grounded had engine trouble.

The man is dead. The police were looking for him.
The man that the police were looking for is dead.
OR The man whom the police were looking for is dead.

You were talking about a woman. She is my boss.
The woman that you were talking about is my boss.