Character jump as3,character falls!

[svdo]

Hello everybody,
i'm new here so not sure whats the étiquette here :P
But i need some help with my actionscript code for a game i'm making for a school project. I think i made it myself a bit too difficult by wanting a character to jump. The problem is that my charcter jumps and walks but when she jumps she doesnt end on the same height as where she started. she falls to the bottom of the page. I think somewhere i can add a code that says an object is the ground but i have no clue how to do that! So someone please help me! So far i have this:
var leftKeyDown:Boolean = false;
var upKeyDown:Boolean = false;
var rightKeyDown:Boolean = false;
var downKeyDown:Boolean = false;
//the main character's speed
var mainSpeed:Number = 10;
noor.addEventListener(Event.ENTER_FRAME, moveChar);
function moveChar(event:Event):void{
//if certain keys are down then move the character
if(leftKeyDown){
noor.x -= mainSpeed;
noor.play();
}
if(rightKeyDown){
noor.x += mainSpeed;
noor.play();
}
if(upKeyDown || mainJumping){
mainJump();
}
}
stage.addEventListener(KeyboardEvent.KEY_DOWN, checkKeysDown);
function checkKeysDown(event:KeyboardEvent):void{
//making the booleans true based on the keycode
//WASD Keys or arrow keys
if(event.keyCode == 37 || event.keyCode == 65){
leftKeyDown = true;
}
if(event.keyCode == 38 || event.keyCode == 87){
upKeyDown = true;
}
if(event.keyCode == 39 || event.keyCode == 68){
rightKeyDown = true;
}
if(event.keyCode == 40 || event.keyCode == 83){
downKeyDown = true;
}
}
//this listener will listen for keys being released
stage.addEventListener(KeyboardEvent.KEY_UP, checkKeysUp);
function checkKeysUp(event:KeyboardEvent):void{
//making the booleans false based on the keycode
if(event.keyCode == 37 || event.keyCode == 65){
leftKeyDown = false;
}
if(event.keyCode == 38 || event.keyCode == 87){
upKeyDown = false;
}
if(event.keyCode == 39 || event.keyCode == 68){
rightKeyDown = false;
}
if(event.keyCode == 40 || event.keyCode == 83){
downKeyDown = false;
}
}
//whether or not the main guy is jumping
var mainJumping:Boolean = false;
//how quickly should the jump start off
var jumpSpeedLimit:int = 15;
//the current speed of the jump;
var jumpSpeed:Number = jumpSpeedLimit;
//jumping function
function mainJump():void{
//if main isn't already jumping
if(!mainJumping){
//then start jumping
mainJumping = true;
jumpSpeed = jumpSpeedLimit*-1;
noor.y += jumpSpeed;
} else {
//then continue jumping if already in the air
//crazy math that I won't explain
if(jumpSpeed < 0){
jumpSpeed *= 1 - jumpSpeedLimit/75;
if(jumpSpeed > -jumpSpeedLimit/5){
jumpSpeed *= -1;
}
}
if(jumpSpeed > 0 && jumpSpeed <= jumpSpeedLimit){
jumpSpeed *= 1 + jumpSpeedLimit/50;
}
noor.y += jumpSpeed;
//if main hits the floor, then stop jumping
//of course, we'll change this once we create the level
if(noor.y >= stage.stageHeight - noor.height){
mainJumping = false;
noor.y = stage.stageHeight - noor.height;
}
}
}
my charcters name is noor.
I appeciate all the help i can get! Thanks!

[Lynkusu]

I don't usually make that type of game, and when I say "don't usually," I mean, "never." But regardless, I'd try making the ground into a movie clip and hitTesting it.

if(noor.hitTestObject(ground)){
[whatever makes her stop];
}

Like that, probably