In everyday speech, the ne of the negative mostly disappears. Pas, rien, jamais and plus carry the meaning on their own:

Careful	Spoken	English
Je ne sais pas.	Je sais pas. → J'sais pas → Chais pas.	I don't know.
Ce **n'**est pas grave.	C'est pas grave.	Never mind.
Il **n'**y a pas de pain.	Y a pas de pain.	There's no bread.
Je **n'**ai rien vu.	J'ai rien vu.	I didn't see anything.
Il ne vient jamais.	Il vient jamais.	He never comes.

This isn't slang: in relaxed conversation almost everyone drops the ne, whatever their background. In writing — emails, reports, exams — and in formal speech, keep it.

Careful with plus: J'en veux plus can mean I don't want any more (plus said “plu”) or I want more (said “pluss”). Only the pronunciation tells them apart.