So i am going to take my first hack at implementing a shopping cart module into an existing site.
My hosting co. comes with 3 options that use the Fantistico auto installer: OSCommerce, CubeCart, and Zen Cart.
Is there a better solution out of those three?
I also need to obtain a merchants account right?
A pity that you said "existing site" as there are many fine - and free - CMS out there which include shopping carts - and many, mnay, many other great features.
If you would consider "porting" your site to one, then I highly recommend visiting the CMS Matrix. Take a look at the free (and already debugged) modules for page cretion, mailing lists, wikis, polls, security, multi-language support ... the list just goes on.
At the site you can also select features which interest you and search for all CMS which offer them and compare the results (the CMS matrix).
I recommend that you at least consider this (and you can't go wrong with Drupal).
And, yes, you will need a merchant accoun to accept credit cards , PayPal, etc (best use SSL too)
Good luck!