Okay, for people to pay you, you have three basic choices:
1.) PayPal. You can sign up for a PayPal business account, and they have instructions on their page for setting up a payment system on your website. Their express checkout is quite easy to set up, and if your site's shopping cart software supports it, their IPN software will notify you immediately of sales. This is the easiest choice, but it has a drawback -- a lot of customers will bail when they see it's PayPal. They have a full website integration checkout that won't alienate customers as much, but it's harder to set up. Also, in my experience I've lost more to fraud from PayPal customers than I've made in profit from them, so I no longer take PayPal for anything but eBay auctions.
2.) Google Checkout. Pretty similar to PayPal, but I've never used it.
3.) Get a real merchant account. For this, if you need your hand held, talk to your bank. You'll need a business checking account to get this. Alternately, you can set it up yourself -- I recommend National Merchant Bancard if you want something cheap. (see
here). Note that "cheap" still means $30/month plus $0.25 + 2.39% per transaction, and you'll need to be using Authorize.net compatible shopping cart software.