Remember we use WHO with people and WHICH for things. We can use THAT for both people and things.
Here you have some exercises for practising:
- http://www.perfect-english-grammar.com/relative-clauses-exercise-1.HTML
- http://www.perfect-english-grammar.com/relative-clauses-exercise-2.html
- http://www.ego4u.com/en/cram-up/vocabulary/who-which