You use ''keine'' when you want to say ''no...'' and ''nicht'' when you want to say ''not''.
EXAMPLE:
Ich sagte ihm,er soll keine Äpfel kaufen.= I said to him,he should buy no apples.
Ich sagte ihm,er soll nicht Äpfel kaufen. = I said to him,he should not buy apples.
( Both are correct,but the first one sounds much better then the second)
I hope you can understand what I mean, my English is not perfect. But it's not a very big difference between ''keine'' and ''nicht''.