namespace LockDoors//name of your script or library
{
    //here you state what libraries of code you are using
    //these are namespaces
    using System;
    using System.Windows.Forms;
    using GTA;

    //this is the beginning of your script or class
    public class LockDoors : Script
    {
        //here u can state variable u would like to use
        //you are not using any so dont worry now
        //look at some other people's source and u see

        //this is the constructor and code runs on script load
        public LockDoors()
        {
            //you are hooking a KeyDown event here, it will look in the method
            //in this case the method's name is LockDoors_KeyDown
            KeyDown += new GTA.KeyEventHandler(LockDoors_KeyDown);
        }

        //this is what u hooked and allows pc to look for key presses
        void LockDoors_KeyDown(object sender, GTA.KeyEventArgs e)
        {
            if (e.Key == Keys.Delete)//so if u press delete then code inside runs
            {
                if (Game.Exists(Player.Character.CurrentVehicle))//this makes sure car exists so u dont do stuff with a null value because your script will crash if something is null
                {
                    if (Player.Character.CurrentVehicle.DoorLock == DoorLock.ImpossibleToOpen)// u check if door is locked here to see how u want key to react
                    {
                        //here the door is locked so your key unlocks
                        Game.DisplayText("Door is Unlocked");
                        Player.Character.CurrentVehicle.DoorLock = DoorLock.None;
                    }
                    else
                    {
                        //here your door is unlocked so the key locks
                        Game.DisplayText("Door is Locked");
                        Player.Character.CurrentVehicle.DoorLock = DoorLock.ImpossibleToOpen;
                    }
                }
            }
        }
    }
}