To accept payments online there are three main factors to be looked at. Here are the following (for an Online merchant account)

1. Payment Gateway. - This entity will transfer the order number of your customer to the merchant account providers and from them as well. The payment gateways will also provide you with the facility to accept billing details like card number, and other specs which are necessary to bill the client.

2. Merchant Account. - This will provide you with the ability to actually accept this information and charge the customer with a particular amount.

3. Website. - This is the first thing which must be up and running before you get your merchant account. All information must be well documented on your website for the user.