Remember to use Who for people and Which for things. That can be used for both.
Using the relative clause that to combine sentences:
- I bought a new winter coat. It is very warm.
- I bought a new winter coat that is very warm.
Using the relative clause which to provide more information about something:
- I live in Los Angeles. Los Angeles has lots of sunshine.
Everyone knows where LA is, so the relative clause provides more information:
- I live in Los Angeles, which has lots of sunshine.
Using who as a defining relative clause:
- I am looking for my brother. He is good at fixing things.
- I am looking for my brother who is good at fixing things