And here, If I may, is another little tip for you:
It is almost imperative for you to follow the guidelines in the tutorials posted by
@Erel above. Let me explain. (Explanation based on the use of an iPhone 6S+ as the dev device)
If you set the device to normal view then the screen size is 414 x 736 - Now you use this as a derivative and design your layout on that - after all - it is an iPhone 6S+ is it not ? You test and everything looks fine and in your mind you think you've got it covered and all set to go.
Your user/client now loads the app on their device, which is an iPhone 6S+ (and remember, you've designed it for an iPhone 6S+ - with fixed screen layout co-ordinates) and
NOTHING fits properly !! - Why is this ?
Your client has probably set the screen to ZOOMED (as opposed to standard) and the layout size is now 375 x 667 !! - Did your client do this intentionally ? Most likely not. They most probably upgraded from and iPhone 5 - whose screen size is ...... - you guessed it !! and for some strange reason the 6S+ takes on the previous iPhones resolution and to do this - sets the new device to ZOOMED. (You can find it under Settings-->Display & Brightness-->View)
Just something that I learnt the hard way and am now paying it forward for other members to benefit from this !!
Enjoy !!