Hi, Linda -
Ask your merchant account provider what payment gateways they support. An example is Authorize.net, but there are many, many others. Once you know what's compatible, you can then look for a VirtueMart payment plugin that supports that gateway. The plugin documentation should guide you through all of the necessary setup.
Two things to consider -
1) You *must* be PCI compliant when you are accepting credit card information on your website. This does not mean just SSL. There is a whole bunch of requirements and standards that you are required to follow to retain your merchant account in good standing with Visa, MasterCard, etc. You can learn more here: https://www.pcisecuritystandards.org/smb/
. My advice is to get the assistance of an experienced ecommerce technical consultant who can help guide you through the process. Doing this wrong - or failing to do it entirely - can be a very costly mistake. Nowadays, with hacking reports being in the news every day, you cannot afford to take chances with your sensitive data (or your customer's credit card information).
2) You might want to check out PayPal PRO. It can help you get up and running faster. Basically PayPal acts as your payment gateway, handling the authorization and routing process between your website, the card holders's bank, and your bank. The fees are a little high but it can take a lot of stress out of getting going with credit cards.
Hope this helps!