hi you can get this details using cart  session

    $totals = Mage::getSingleton('checkout/session')->getQuote()->getTotals(); //Total object
    $subtotal = $totals["subtotal"]->getValue(); //Subtotal value
    $grandtotal = $totals["grand_total"]->getValue(); //Grandtotal value
    if(isset($totals['discount']) && $totals['discount']->getValue()) {
     $discount = Mage::helper('core')->currency($totals['discount']->getValue()); //Discount value if applied
    } else {
     $discount = '';
    }



    
    if(isset($totals['tax']) && $totals['tax']->getValue()) {
     $tax = $totals['tax']->getValue(); //Tax value if present
    } else {
     $tax = '';
    }

If you want to item discount check  this

    $quote = Mage::getSingleton('checkout/session')->getQuote();
    
    Then, to get the list of items in the cart :
    
    $cartItems = $quote->getAllVisibleItems();
    
    Then, to get the count for each item :
    
    foreach ($cartItems as $item) {
        echo $item->getQty();
    $item->getPrice();
    $item->getDiscountAmount();
    }

more details to check table sales_flat_quote_item in your magento system