The first question is probably the most significant. If you are tech-savvy and feel comfortable setting up your own shopping cart, you will be able to take advantage of one of the many free, open-source carts out there. But if you need a hosted solution, that will take you in another direction entirely.
osCommerce is one of the oldest and most respected free, open-source solutions. It will work on most any PHP-enabled Web server, and will work with most of the popular payment gateways. There are many other open-source solutions, including ZenCart and MyMarket.
There are also commercial solutions you can buy and install yourself, but I highly recommend the open-source programs. Open-source applications are generally very secure, have active development and support communities, and they're free!
If you aren't comfortable with the do-it-yourself approach, there is certainly no shortage of paid solutions. If you are a true technophobe, a fully hosted application is probably your best bet. StoreFront is one such option. It offers everything from site design templates through payment processing, all with unlimited 24-hour tech support. StoreFront can also automatically import transaction information into QuickBooks to save you from having to enter it manually.
Microsoft and Yahoo both have cart offerings that are popular with small businesses. Microsoft's offering, called Commerce Manager, and Yahoo's, called Merchant Solutions, can have you online and accepting orders quickly.
But if you need to start accepting payments today, PayPal may be the way to go. You can sign up for an account in minutes, and PayPal's cart application is free. Once your account is activated, you can start accepting bank transfers, e-checks, credit card payments, and payments from your customers' PayPal accounts. PayPal's fees are a little higher than most other big merchant account processors, but the convenience is worth it for many vendors.
With so many cart options out there, you are bound to find one that fits your needs.