PFQuery *query = [PFQuery queryWithClassName:@"YourClassName"];
[query orderByDescending:@"createdAt"];
[query getFirstObjectInBackgroundWithBlock:^(PFObject *object,NSError *error) {
    // code
}];

在斯威夫特：

var query = PFQuery(className: "YourClassName")
query.orderByDescending("createdAt")
query.getFirstObjectInBackgroundWithBlock {(object: PFObject?,error: NSError?) -> Void in
    // code
}