I assume you are talking about in the display of your products in a specific category? If so, you will want to edit products.tpl. The buy now button is this piece of code:
Code:
{if $usertype eq "C" and $config.Appearance.buynow_button_enabled eq "Y"}
{include file="customer/main/buy_now.tpl" product=$products[product]}
{/if}
Just cut that and paste it where you want it. If you want to put it next to the description, you will probably want to split the cell containing the description into two cells, with the description on the left and the buy now include on the right.