For proper preparation, consult 'Caravan Publishers Encyclopedia of General Knowledge' by Ch. Najib and, PPSC Past Papers published by Advanced Publishers by Imtiaz Shahid. These two books are sufficient. For online preparation, I don't think the method can award success as one has to study properly and extensively from the books as it is MCQs based general knowledge paper.
For Urdu portion, one should get know-how of the important books or authors of Urdu Literature. But, the above-mentioned books also comprise Urdu section and that is almost enough.

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"But screw your courage to the sticking place,
And we'll not fail." _Shakespeare, 'Macbeth')