How do I make my game fit perfectly to all screens?


So , I am making a game and when I exported it , it didnot fit to the screen of my mobile(android).So how do I make it fit to the screen ?? I want my game to stretch to fit in the length of my game! I dont know if activating full screen helps but on my PC in preview mode, the full screen activation makes very hard to cancel out of the game/exit the game! Please help me! I want my game to fit perfectly for all mobile devices leaving no blank black spaces on the sides!