how to make an fps character crouch

ive been working on a fps game for the past few days but I don't know how to make my character crouch please can you help me. by the way I'm quite new to unity so please don't make the answer to complicated.

There's a whole bunch to consider as far as what else your character is doing (can you jump when crouched? Is it different than jumping otherwise? How is it different? Does your movement speed change when crouching? etc.). You don't include any information about your setup such as your camera setup, your character setup, whether you are using a CharacterController, do you have animations, etc. The simple approach (while making some broad assumptions about what you are doing because you don't describe at all) is something like:

var crouching : boolean = false;
var altHeightChange : float = 3.0f;
private var controller : CharacterController;

function Start() {
    controller = GetComponent(CharacterController);
}

function Update() {
    if(controller && Input.GetButtonUp("Crouch")) { //Crouch on/off 
        crouching = !crouching;
        if(crouching) {
            controller.center.y -= altHeightChange * 0.5f;
            controller.height -= altHeightChange;
            //move your camera down by altHeightChange
            animation.CrossFade("crouch");
        }
        else {
            controller.center.y += altHeightChange * 0.5f;
            controller.height += altHeightChange
            //move your camera up by altHeightChange
            animation.CrossFade("stand");
        }
    }
}

If you wanted crouch only when the key is held down, something like this:

var altHeightChange : float = 3.0f;
private var controller : CharacterController;

function Start() {
    controller = GetComponent(CharacterController);
}

function Update() {
    if(controller) {
        if(Input.GetButtonDown("Crouch")) { //Crouch on
            controller.center.y -= altHeightChange * 0.5f;
            controller.height -= altHeightChange;
            //move your camera down by altHeightChange
            animation.CrossFade("crouch");
        }
        if(Input.GetButtonUp("Crouch")) { //Crouch off
            controller.center.y += altHeightChange * 0.5f;
            controller.height += altHeightChange
            //move your camera up by altHeightChange
            animation.CrossFade("stand");
        }
    }
}

If you have more states than crouching or standing, such as prone or jumping, etc. you might consider using and enum for your state.

You might want to take a look at this post too:

http://answers.unity3d.com/questions/14803/how-to-make-the-fps-character-crouch