Basically, whenever you want to change a sentence into its negative form you will usually have ne...pas within the sentence. This is not always the case, however.
For example, to say: 'he is not going' in its absolute simplest form would be "il ne va pas". However, you could also say, "il ne va jamais" which is also a negative phrase that translates to "he is never going".
In informal speak, you may sometimes drop the 'ne'. For example: Je sais pas instead of Je ne sais pas.
Anyways, that is slightly more advanced, but I'm guessing you are quite new to French?
In that case, I would consider simply mastering the ne...pas.
For the example you have provided, the answer would be: Je ne travaille pas le samedi matin. (I don't work on Saturday morning)
Work through some more examples:
Tu vas au parc aujourd'hui.
Je pense que il veut partir.
Il va mourir.
Remember to place the "ne" before the conjugated verb, and "pas" right after the verb.