Hello Issac,
if you take a look at the product details page, you will see there are two default options (as set in the admin), when adding an item to the cart: a) stay on the page or b) go to the cart.
You can set your store to use the 'go to cart' option, and then change the code so it actually goes to checkout. This should be a quick, easy fix.
(if you only want this behavior on some items, you would have to add a little more conditional logic to the process, but that should be fairly straightforward as well)