Merchant Account or Third Party Processor

The quick and dirty way is to setup a Paypal account and create a payment page that you can (sort of) customize and link to from your site. The problem with that approach is that it will not look/feel exactly like the site you're building, the purchase pages are not in the same domain (e.g., www.yoursite.com), and the PayPal process requires that purchasers use paypal for purchasing in some cases which will turn people off.

Check out this e-commerce site and try to buy something:
http://www.kalaharitea.com

You'll notice that the cart and checkout pages look exactly like the main site and the process is quite seamless.

To get that effect, you'll need to buy and implement a shopping cart or e-commerce platform and connect it to a payment processor (e.g., Authorize). I wouldn't try to build one at this point in the development of the web, as there are many packages out there than can handle payments.

If you need recommendations of e-commerce platforms, don't hesitate to contact me.

PAYPAL but I don't recommend it, because the can lock/freeze your account (no withdrawing of funds) for no reason and you have to provide information to satisfy them before they release your funds, the can also close your account and freeze the funds, It's highly recommend that you get a real merchant account, there are a few good ones which you can sign up for online, meaning, when someone pays you, it goes straight into your bank account, you have more control over your money BUT the problem is shopping cart integration

PAYPAL works great with OS Commerce, from my experience.
But you can also manually process the credit card using your own merchant account but the easier way is PAYPAL definately, I use ALERTPAY now, similiar to paypal as well. Also can be integrated into OS Commerce & Zencart.