We have no account of his argument. We do have Plato's account of Socrates' cross examination. Socrates sums up Meletus' accusations as: corrupting the youth and not believing in the gods of the state.

We have five reports that I can find just now. The first four are not independent of one another: they're reports given by Sokrates' students (Xenophon, Plato) or derived from the same material (Diogenes Laertios). The fifth is independent from the first four.

Xenophon, Apology 10:

• that Sokrates did not believe in (nomisd-) the gods that the state venerated, but introduced (eispher-) new divinities (daimones);
• that Sokrates corrupted (diaphtheir-) the young.

Plato, Apology 24b-c:

• that Sokrates did injustice by corrupting the young;
• that Sokrates did not believe in the state gods, but in new divinities (daimonia).

Plato, Euthyphron 2c-3b:

• that Sokrates corrupted the young;
• that Sokrates made (poie-) new gods (theo-) and did not believe in the old ones (tous... archaious).

Diogenes Laertios 2.40 (claiming to give the exact text of the charge):

• that Sokrates did injustice by not believing in the gods in whom the city believed, but introduced (eisēge-) new divinities (daimonia);
• that Sokrates did injustice by corrupting the young.

Aischines Against Timarchos 173:

• that Sokrates was shown to be the teacher of (paideu-) Kritias, the most notorious of the Thirty Tyrants who usurped the democracy in 404 BCE, killed a significant percentage of the citizen population, and stole their property.