Read some tutorials.
Need sprites, make them objects, give controls to objects, give instructions on what happens when object collides another.
For Example, Snake and a wall.
Find or create a sprite of a snake and a wall.
Create object and place sprite as the visual.
Give the snake object controls.
Turn on collisions for snake and wall.
There should be a menu saying "Collides with" and set it to snake and wall.
Have it action End Game.
Thats pretty vague, i havnt done it since year 10, thats two horribly long years.