I have been learning JavaScript and Unity for a week now. I learned how to make Cube as a Ground and another Cube as a player and I used this code to make the Player Cube move forward and backward and jumping

var walkspeed: float = 5.0;
var jumpheight: float = 250.0;
var grounded = false;

function Update() {

    rigidbody.freezeRotation = true;

    if (Input.GetKey("a")) 
       transform.Translate(Vector3(-1, 0, 0) * Time.deltaTime * walkspeed);
    if (Input.GetKey("d"))
       transform.Translate(Vector3(1, 0, 0) * Time.deltaTime * walkspeed);

    if (Input.GetButton("Jump")) {
        Jump();
    }
}

function OnCollisionEnter(hit: Collision) {
    grounded = true;
}

function Jump() {
    if (grounded == true) {
        rigidbody.AddForce(Vector3.up * jumpheight);
        grounded = false;
    }
}

I also learned how to make a character hit box. how to make a sprite and animation. pretty much the basic stuff.

Couple of days ago I created simple ground in Photoshop and a simple character and imported them to Unity3D. Whenever I use my code above the character keeps falling from the scene. Like the character has nothing to stand on. After thinking about it it make sense because I really didn't make anything to make the player character understand that he should stand on something so I started reading about this issue and I realized that there is something called Collider2D and Rigidbody2D.

Now I'm really stuck here I just don't know what to do. I applied the rigibody2d to my character picture and the Collider2D to the ground picture but whenever I play the project the gravity makes my character falls down.

This is my question:

How can I make the rigibody2d object realize that it shouldn't fall if there is a Collider2D object under it? So when I jump it's going to jump and the gravity going to bring it back to the ground.