One of the standards is the "Babel text" or the story of the tower of Babel from the Bible. That's Genesis 11: 1-9. I am going to link that from Bible Gateway because you can switch between English translations there pretty easily, and that can be useful in deciding how to translate it yourself. LINK OF BABEL
If you aren't into the Bible so much, you might try The North Wind and the Sun. It is one of Aesop's fables and there are several slightly different versions floating around out there. Here's one of them: NORTH WIND AND THE LINK
Looking for some sentences of increasing complexity to have a go at? How about Graded Sentence for Analysis? SENTENCES FOR LINK
If that's not your cup of hot caffeinated beverage, you might try the American Book Review's 100 Best First Lines from Novels: NOVELLINK
But what if you're well beyond translating sentences or short passages? Project Gutenberg to the rescue! Two popular choices are Alice's Adventures in Wonderland and The Tale of Peter Rabbit. If that's too juvenile for you, perhaps you could try Metamorphosis -- nothing is more Kafkaesque than Kafka. If that's not your style, perhaps you'd like to have a stab at Pride and Prejudice. Still not your style? Check your bookshelves! You've undoubtedly got something good there!