How do you write a realistic scene?

I'm writing a performance and whilst I'm pretty good at doing the monologue-type parts, I'm not that great at doing the everyday dialogue. Whenever I am trying to make a conversation sound natural it just feels forced. There is one scene in particular where the father of two teenagers has just died and they're grieving for him. I need to make it sound realistic without being overly mushy or completely fake sounding. Any suggestions? :)