What do you say?

What do you say to the coworkers whos son is in the hospital for the second time for the same thing or to another coworker who has some kind of infection and is in the hospital and what about the friend whos brother passed away?
Really I have never been good at these things. I think some great things in my head but they NEVER come out of my mouth the same way as they did in my head and usually I end up saying some thing that is TOTALLY not right.