to get you started, the easiest way to do this is go to Insert-->Webassist-->eCart-->Display Manager
From the pop-up. choose the display type 'quick cart'
this will display the number of items and the subtotal from the ecart array.
if you want to dig a bit deeper, look at the code, and you will see that the number of items and the subtotal are as follows, held in a table...
<td class="eC_QuickCartQuantity">Items in cart: <strong><?php echo $cartname->TotalColumn("Quantity"); ?></strong></td>
<td class="eC_QuickCartTotal">Total: <strong><?php echo WA_eCart_DisplayMoney($cartname, $cartname->GrandTotal()); ?></strong></td>
where $cartname is the name of your cart instance.
if that is all you want, you can delete the superfluous button code