We use who when we talk about people.

I don't know who told you that.

We use which or that when we talk about things.
Which is more formal than that.
Which and that can be left out of a defining relative clause.

It's the watch which my husband bought me for my birthday.
It's the watch that my husband bought me for my birthday.
It's the watch my husband bought me for my birthday.

We use whose to show possession.

John, whose brother was also a musician, plays over 100 concerts every year.

We use where / when when we talk about place or time.

My wife and I went to the bar where we first met.
It was when I was living in London.