How do I add gravity to my object, and how do I fix my network problem?

Hello,

I am trying to create a small game, to start with, in Unity3D. This is supposed to be a basic game that works over a network. So far, I have my network working, and I had it where one player could see another player moving, and vice versa. Here are my problems: 1) I don't know how to add gravity to my cube (player). Using Rigidbody has not worked, because it's caused my cube to fly all over the screen. 2) I don't know how to fix the problem I am having with my network. At the moment, when one player moves his or her character, it actually moves the other player's character, and vice versa.

You can view this YouTube video to see my problems in action.

Also, when I add gravity to the player/character in another test game (not the one in the video), it tips over based on the slope of the terrain. I want the character (which is a 2x1x1 cube) to stand upright, regardless of the terrain setting.

Here is some code from the game that is shown in the video:

Movement.js Code:

 var speed:float = 0.2;
 var rotateSpeed:float = 5;
 
 function Start(){
     if(!networkView.isMine)
     {    
         enabled = false;
     }
 }
 
 function Update(){
         if(Input.GetKey(KeyCode.UpArrow))
         {
             transform.Translate(Vector3.forward * speed);
         }
         if(Input.GetKey(KeyCode.DownArrow))
         {
             transform.Translate(Vector3.back * speed);
         }
         if(Input.GetKey(KeyCode.RightArrow))
         {
             transform.Rotate(Vector3.up * rotateSpeed, Space.World);
         }
         if(Input.GetKey(KeyCode.LeftArrow))
         {
             transform.Rotate(Vector3.down * rotateSpeed, Space.World);
         }
 }

NetworkManagerScript.js Code:

 var playerPrefab:GameObject;
 var spawnObject:Transform;
 
 var gameName:String = "CJay7_Test";
 
 private var refreshing:boolean;
 private var hostData:HostData[];
 
 private var buttonX:float;
 private var buttonY:float;
 private var buttonWidth:float;
 private var buttonHeight:float;
 
 function Start(){
     buttonX = Screen.width * 0.05;
     buttonY = Screen.width * 0.05;
     buttonWidth = Screen.width * 0.15;
     buttonHeight = Screen.width * 0.15;
 }
 
 function startServer(){
     Network.InitializeServer(32, 25001, !Network.HavePublicAddress);
     MasterServer.RegisterHost(gameName, "Test Game", "I am creating a test game from a tutorial. 'http://vimeo.com/33996023#'");
 }
 
 function refreshHostList() {
     MasterServer.RequestHostList(gameName);
     refreshing = true;
 }
 
 function Update(){
     if (refreshing)
     {
         if (MasterServer.PollHostList().Length > 0)
         {
             refreshing = false;
             Debug.Log(MasterServer.PollHostList().Length);
             hostData = MasterServer.PollHostList();
         }
     }
 }
 
 function spawnCharacter(){
     Network.Instantiate(playerPrefab, spawnObject.position, Quaternion.identity, 0);
 }
 
 //MESSAGES
 function OnServerInitialized(){
     Debug.Log("Server Initialized");
     spawnCharacter();
 }
 
 function OnConnectedToServer(){
     spawnCharacter();
 }
 
 function OnMasterServerEvent(mse:MasterServerEvent){
     if (mse == MasterServerEvent.RegistrationSucceeded)
     {
         Debug.Log("Registration Succeeded");
     }
 }
 
 //GUI
 function OnGUI(){
     if(!Network.isClient && !Network.isServer){
         if (GUI.Button(Rect(buttonX, buttonY, buttonWidth, buttonHeight), "Start Server"))
         {
             Debug.Log("Starting Server");
             startServer();
         }
 
         if (GUI.Button(Rect(buttonX, buttonY * 1.2 + buttonHeight, buttonWidth, buttonHeight), "Refresh Hosts"))
         {
             Debug.Log("Refreshing");
             refreshHostList();
         }
         
         if (HostData)
         {
             for (var i:int = 0; i < hostData.length; i++)
             {
                 if(GUI.Button(Rect(buttonX * 2 + buttonWidth, buttonY * 1.2 + (buttonHeight * i), buttonWidth * 3, buttonHeight * .5), hostData[i].gameName))
                 {
                     Network.Connect(hostData[i]);
                 }
             }
         }
     }
 }

Again, please view this YouTube video to see my problems in video form. Thank you in advance for any constructive and helpful comments.