Use who for people and which for things; that can be used in many defining relative clauses. The clause identifies the person or thing you mean.
Practical examples
The person who called is my colleague.
الشخص الذي اتصل هو زميلي.
This is the book that I mentioned.
هذا هو الكتاب الذي ذكرته.
We need a room which has a window.
نحتاج غرفة فيها نافذة.