The words that and which point to something — which one? That one! Before a clause or phrase, a that clause goes with the flow, but a which clause starts with a pause. American English makes a big deal out of the distinction but British English doesn't, which may be why it's so dang confusing.

There's no comma before that because the info that comes after is important. That restricts the meaning. For example, "Please grab the coat that makes her invisible" is not the same as "Please grab the coat." She needs to be invisible! Here are some examples, and read the rest of the nursery rhyme if you love thats:

"This is the house that Jack built." (Nursery Rhyme)

"You're the One That I Want." (Grease)

Which, on the other hand, is like, "oh, by the way." A which begins a nonrestrictive clause or phrase, meaning it doesn't restrict the meaning of the sentence and needs a comma. So if you say, "Please grab the coat, which she left on the floor," use which if it doesn't really matter where she left it. Just grab the coat already. Like the bad witch from the Wizard of Oz, words beginning with which are nearly expendable, like in these examples:

"So we do not need to step out of the house for days, which I am so looking forward to." (Oxford English Dictionary)

"Ed's house, which is located on thirteen acres, was completely furnished with bats in the rafters and mice in the kitchen." (Rules for Writers)

So if you use that, don't use a comma because it needs to flow with the rest of the sentence. But a which whispers in your ear and deserves a little pause, which is kind of cool.