How do I make a semi-transparent view layer above a current view?

// get your window screen size
CGRect screenRect = [[UIScreen mainScreen] bounds];
//create a new view with the same size
UIView* coverView = [[UIView alloc] initWithFrame:screenRect];
// change the background color to black and the opacity to 0.6
coverView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.6];
// add this new view to your main view
[self.view addSubview:coverView];

when you are done with it , you can remove it :

[coverView removeFromSuperview];

Swift3 version:

// get your window screen size
let screenRect = UIScreen.main.bounds
//create a new view with the same size
let coverView = UIView(frame: screenRect)
// change the background color to black and the opacity to 0.6
coverView.backgroundColor = UIColor.black.withAlphaComponent(0.6)        
// add this new view to your main view
self.view.addSubview(coverView)

to remove it:

coverView.removeFromSuperview()

This can be easily accomplished with a UITextView and a UIButton. Simply place the UITextView on the screen with the content you want to display, making it the full frame size of the screen, and change the background color to a black background with background alpha of .6

[UIColor colorWithRed: 0 withGreen: 0 withBlue: 0 withAlpha: .6];

Then place the button as a subview on top, making it the full frame of the screen, and setting it's alpha to 0. Set the action of the button to hide the textview and button.

Example:

UITextView* textview = [[UITextView alloc] initWithFrame:self.view.frame];
[textview setText: @"Text here"];
[textview setBackgroundColor: [UIColor colorWithRed: 0 withGreen: 0 withBlue: 0 withAlpha: .6]];
[textview setTextColor: [UIColor whiteColor]];
[self.view addSubview: textview];

UIButton* btn = [[UIButton alloc] initWithFrame:self.view.frame];
[btn setAlpha:0];
[btn addTarget:self action:@selector(method) forEvent:UIControlEventTouchUpInside];
[self.view addSubview: btn];

You may want to check the addTarget: method; I'm not sure those are the correct parameters off the top of my head.

Just make a mechanism that figures out if it is the first launch of the app, then tells your main view controller to add a (probably image) view on top of the current view with 0.6 alpha

if (figureOutIfIsFirstLaunch)
{
    UIImageView *myOverlay = [...];
    myOverlay.frame = self.view.bounds;
    myOverlay.alpha = 0.6;
    [self.view addSubview:myOverlay];
}