please check the Mage_Checkout_CartController and addAction() inside, you will find everything you need ;)
$cart = $this->_getCart();
$params = $this->getRequest()->getParams();
try {
if (isset($params['qty'])) {
$filter = new Zend_Filter_LocalizedToNormalized(
array('locale' => Mage::app()->getLocale()->getLocaleCode())
);
$params['qty'] = $filter->filter($params['qty']);
}
$product = $this->_initProduct();
$related = $this->getRequest()->getParam('related_product');
/**
* Check product availability
*/
if (!$product) {
$this->_goBack();
return;
}
$cart->addProduct($product, $params);
if (!empty($related)) {
$cart->addProductsByIds(explode(',', $related));
}
$cart->save();
$this->_getSession()->setCartWasUpdated(true);
/**
* @todo remove wishlist observer processAddToCart
*/
Mage::dispatchEvent('checkout_cart_add_product_complete',
array('product' => $product, 'request' => $this->getRequest(), 'response' => $this->getResponse())
);
if (!$this->_getSession()->getNoCartRedirect(true)) {
if (!$cart->getQuote()->getHasError()){
$message = $this->__('%s was added to your shopping cart.', Mage::helper('core')->htmlEscape($product->getName()));
$this->_getSession()->addSuccess($message);
}
$this->_goBack();
}