How do I prevent the steering wheel from locking when I turn off the engine?
That's just the way it is, you can remove the locking system but it is really involved and as it is a safety item a shop won't touch it. Try moving the steering wheel slightly when in the lock position as you turn the key, it might help some.
Since this vehicle is only one year old take it back to the Toyota Dealer. If anything is wrong it will be covered by the new car warranty. HTH. -Jim