Hi! I would say, start at the bank where you/your clients do business. While banks are often a big ripoff, it helps to have a rate for comparison and sometimes banks will lower the rates if you really haggle.
Banks will make it sound like the percentages and fees they will charge you for a merchant account with a credit-card scanning unit are set in stone - really it's totally capricious. Pretend you are at a Persian Rug market, haggle and play hardball, it works.
There are now a plethora of online merchant services - you can google for their Web pages. I would suggest that you NOT go with what's cheapest. Make sure that you can speak with a live human for support. Lots of $$ and customer satisfaction will be in the balance, make sure you won't be fumbling around with a customer's $$ when systems go "down", etc.
Getting a merchant account is very difficult for many, it's not unlike getting a morgage. Before you can start playing with credit cards they will look at your books, licenses, credit, banking histories and even talk to your friends! No kidding. So just be sure you have got it together before invoking these companies (some of the most powerful in the world, remember.)
If that can't happen, then explore some of the all-in-one online services, especially since you are selling digital products and not widgets needing to be shipped, etc. Look for a company that has "buy now" buttons and will process the back-end of the transaction. Usually for a fairly significant percentage plus fees.
I know some sellers that are having good success taking their wireless laptop to the marketplace and having customers standing at the booth use PayPal buttons on their Web site to sell stuff, then just handing it to the customer. Pretty cool. Inexpensive approach.