How to call a View Controller programmatically?

To create a view controller:

UIViewController * vc = [[UIViewController alloc] init];

To call a view controller (must be called from within another viewcontroller):

[self presentViewController:vc animated:YES completion:nil];

For one, use nil rather than null.

Loading a view controller from the storyboard:

NSString * storyboardName = @"MainStoryboard"; 
UIStoryboard *storyboard = [UIStoryboard storyboardWithName:storyboardName bundle: nil];
UIViewController * vc = [storyboard instantiateViewControllerWithIdentifier:@"IDENTIFIER_OF_YOUR_VIEWCONTROLLER"];
[self presentViewController:vc animated:YES completion:nil];

Identifier of your view controller is either equal to the class name of your view controller, or a Storyboard ID that you can assign in the identity inspector of your storyboard.