SHORT ANSWER:
Authorize.net and Paypal are the most popular gateways, and therefore have the most community support. To find a processor, I would use http://www.transfs.com/. We used TransFS last year and they helped us find an inexpensive processor.

LONG ANSWER:
The number of players you have to go through in order to process credit cards is pretty crazy. Here's how I'd summarize the different people you might have to deal with.

1. There are credit card companies, like Visa, Mastercard, and American Express. You're affected by their fees and policies. For example, we found out that Visa has been enforcing a policy where they don't allow the capture amount to be different from the auth amount, except for merchants that get tips like restaurants and salons.

2. There are credit card processors, also known as ISOs (for Visa) and MSPs (for Mastercard). They are responsible for communicating your charges to Visa and Mastercard. They're also responsible for figuring out how risky you are as a vendor. If you charge $100, and the buyer claims that the $100 charge was fraudulent or unsatisfactory (also known as a chargeback), and then you go bankrupt and refuse to remit the $100, the credit card processor is stuck with the bill.

3. Credit card processors use gateways like Braintree and Authorize.net to interface with your software. It's much easier for them to use a well-known API than writing their own API.

4. Merchant accounts are accounts where your money get stored immediately after the merchant charges the clients' cards. For most small merchants, the way it works is that the credit card processor has a merchant account on the merchant's behalf, and the funds get deposited from the (credit card processor controlled) merchant account into the merchant's checking account 24 to 48 hrs after the transaction. In other words, if you're just starting out, I don't think you need to worry about getting a separate merchant account from a bank. However, if the merchant is big enough that the float on the funds during those 24 hours actually matters, he or she might get his own merchant account.

5. The process for applying to a credit card processor (2) is fairly complicated. There are people who help you with this - you can call them agents. These agents get paid by the credit card processors for bringing them business.

6. Finally, another tier has emerged recently to help agents attract business - we can call them agent referrers. Agent referrers help you figure out credit card terms and will refer you to both independent agents (5) and credit card processors (2) like Paypal. They endeavor to make the process transparent for you.

We found an agent (5) and a processor (2) through an agent referrer (6) and were quite happy with the process. The referrer was www.transfs.com, and the agent turned out to be www.cocard.com. Transfs.com was extremely clear about the fees we'd incur. I'm not even remotely affiliated with either company except as a client. Before that, we
tried a couple different agents and we also tried contacting our bank to see if they could set us up with a merchant account, but both of those approaches were dead ends.

I hope that all helps.