commands -> tu or vous?

I'm wondering about commands in French. I was taught to command in vous-form, but I know that you can also command in tu-form, so I'm just wondering what the difference is?

In particular, what I'm trying to say is "choose joy" and I'm just wondering if it would be more proper to say "choisis la joie" or "choisissez la joie."

Of course, I haven't studied French for two years now and it's quite possible that neither of these is quite right, but I'm just wondering how to know the difference between the two AND how to properly say this exact phrase.