French negation can be tricky. Normally, to make a statement negative you need to surround the conjugated verb with the French pair ne... pas. However, if you've ever watched French movies or television, or chatted with native speakers, you have almost certainly heard pas (or another negative adverb) without ne, as this is a typical characteristic of informal and familiar French. Although it is nearly always written, ne is often dropped in spoken French.
Purists will tell you that this is wrong (and they tell me I shouldn't teach it), but the reality is that this is how the French speak, so if your goal is to sound more French, it's how you should speak too.
Je ne sais pas > Je sais pas.
I don't know.
Il ne va pas venir > Il va pas venir.
He isn't going to come.
Elle n'est pas encore arrivée > Elle est pas encore arrivée.
She hasn't arrived yet.
Ne bouge pas ! > Bouge pas !
Il ne faut pas faire ça ! > Il faut pas faire ça !
You shouldn't do that!
Note: It's not just ne... pas that loses the ne - all the other negative structures do as well.
Je n'ai plus d'argent > J'ai plus d'argent.
I don't have any more money.
Nous ne le voyons jamais > Nous le voyons jamais.
We never see him.
Je n'ai aucune idée > J'ai aucune idée.
I have no idea.
Je n'en sais rien > J'en sais rien.
I don't know anything about it.