It's partly an admin setting and partly dependent on whether your products have attributes.
If a product has attributes, choices need to be made before purchase. These can only be handled in the product info page, so there cannot be a buy now button on the product listing page for that product.
By default, there is either a "... more info" or an "add to cart" button on every product in listing pages, so if yours is missing, you or the maker of your template have disabled it. This can be turned on/off in admin.
For the product listing, look in Configuration > Product Listing > Display Product Add to Cart Button (0=off; 1=on; 2=on with Qty Box per Product)
and
Display Multiple Products Qty Box Status and Set Button Location.
For featured/all/etc listings, see the corresponding page under Configuration, and set
Display Product Buy Now Button
or
Display Multiple Products Qty Box Status and Set Button Location.