How do I assign a prefab to a variable with a script?

I have variable whose prefab keeps changing to none each time i start the game. How do I set the prefab through the script?

what i can understand from the question can be done as:-

 void Start()
{
       GameObject your_variable = Resources.Load(".. file path in resource") as GameObject;
}   

I am not sure if you are using a GameObject or Mesh or whatever type. Use it accordingly