There are many ways you can go about accepting credit cards for transactions on your site. The most common include:

• using an internet gateway to process your transactions, such as Authorize.net, Linkpoint, Verisign or Echo.
• using PayPal or another third party services to process your transactions
• Merchant's Accounts and Third Party Processor?
• Merchant Account

With a merchant's account you'll have a monthly cost that include access to the service, statement fees, plus a percentange and/or fee charged per transaction. The monthly cost is something you pay, whether you use the service or not. Figure around $50/month - your exact cost may vary.
With a merchant's account you get the ability to seamlessly take credit cards from your site. This can usually be integrated directly into your shopping cart, so your visitor never leaves your site during the transaction.  You also have the ability to manually process transactions via login to your merchant's account website.

PayPal, Google Checkout or Others

With options like PayPal, you don't have the monthly costs associated with the merchant's account. You pay a percentage on each transaction, that's it. This can be a very cheap and easy way to get started.

• Charge.com - offers fast approvals and activation.