Different merchant accounts take different settings to set them up. It really depends on the gateway. I'm not out to start a war about which cart is the best but osCommerce has many gateways and if it's not there by default there's probably a contribution to support that merchant gateway.
Alot depends on that Merchant account that your friend can set you up with.
Most important is what Processor they're connected to. Don't mix-up processor with gateway. Gateway's like Authorize.net and ValetPay.com are connected to the processors. For example, we are connected to the Vital and NOVA processors. Any merchant account that uses either of these processors is compatible with our gateway service. Authorize.net is compatible with at least 6 different processors.
Once you've determined which gateway you can/want to us, then you'll want to find a shopping cart software that works with the gateway. As the previous post recommends, I too recommend osCommerce. It works with most gateway's.
Additionally, many/most merchant accounts require that your website has the following information posted on the website. Your friend has probably already notified you of these requirements.
. returns policy
. pricing of products