Have you checked out Authorize.net's documentation? It's pretty clear - but you'll need some method of sending the info to them in the appropriate format, of generating a unique order id, and receiving/recording the response back from them. They have several pdf's outlining the process, with examples, I think you have to be logged in to your account to see the links.
If you have a shopping cart, it should be fairly simple. SIM is the simplest method they have. Hope this helps.