Recommended translation: "Critias," trans. D. Clay in Plato: Complete Works, ed. J. M. Cooper (Hackett, 1997).

Excerpt:

Timaeus
How gladly do I now welcome my release, Socrates, from my protracted discourse, even as a traveller who takes his rest after a long journey! And I make my prayer to that God who has recently been created by our speech (although in reality created of old), that he will grant to us the conservation of all our sayings that have been rightly said, and, if unwittingly we have spoken aught discordantly, that he will impose the fitting penalty. And the correct penalty is to bring into tune him that is out of tune. In order, then, that for the future we may declare the story of the birth of the gods aright, we pray that he will grant to us that medicine which of all medicines is the most perfect and most good, even knowledge; and having made our prayer, we deliver over to Critias, in accordance with our compact, the task of speaking next in order.

Online:
Amazon (Recommended Translation)
Project Perseus (English, Free Access)
Project Perseus (Greek, Free Access)
Project Gutenberg (Free Access)