Authorize.net is your payment gateway, this is only half of the battle. You'll need a merchant account as well which will allow you to actually collect the funds through the gateway. Authorize.net is owned by Visa and is the industry standard, well as much as possible anyway. The gateway includes sufficient documentation and a large community base to assist you with troubleshooting issues.
In terms of the merchant account, I would start by going to your bank. The one where you do you daily banking for the business. As you have a relationship with them, they'll want to be competitive in pricing. Gather as much information as you can and ensure that they support Authorize.net (which they probably do). Then do a quick online search for merchant account providers. You'll be overwhelmed by the number of online providers for a merchant account. Gather the pricing and then go back to your bank. More often than not, your local bank will match the pricing you find elsewhere. In this way, you get the advantage of dealing with a local institution that you already have a relationship with and still get the best price available.
When it comes to processing with Authorize.net you will need to decide if you're going to stay outside of PCI scope or venture into the dark world of PCI compliance. Many new eCommerce players brush off the idea of PCI and then get nailed or worse, shut down when something goes wrong.