How do you comfort people?

i know i know, it sounds like a pretty stupid question. but seriously, i don't know how to comfort someone when they're feeling down or crying. like today, my friend broke down to me, and i knew what she was going through but i didn't know what to say. i mean, when do you know the right time to rub their back, or hug them? and what do you say? the only thing i'm good at is making people laugh so i did that to make her feel better but i felt pretty sh*tty after b/c i couldn't do nothing about her situation. i feel like i temporary helped her through the moment. i nvr know how to deal with the deep stuff. please help, i've never been good at this..