There two ways in which you can accept credit cards on your website. First is using your own Merchant Account, you can do this by opening a merchant account from your local bank. Requirements are provided by your bank and may vary from country to country. Second is through a Third Party Merchant, they will take care of the online payment processing and send to you the total sales less the commissions and transaction fees and receive it through checks or bank wire transfer.
The costs of opening your own merchant account is higher than using a third party merchant. Most of the third party merchants have no setup or activation fee. They earn from commissions based on your sales.
If you are just starting your ecommerce business and considering your business as small to medium size, use the Third Party Merchant, so you can focus on product strategy, marketing and customer service part. For big time businesses you may have your own merchant account in order to build more reputation and professionalism.