Credentialing is where you contract with carriers to become participating. It involves obtaining all the licensing and credentials of the provider(s). Each carrier has their own way of doing things, so it's always best to check with them for their procedures. Many will want what's called a "letter of intent" which is exactly what it means.. letter of intent to join network. If a network is closed, they will usually retain the Letter of intent for a certain time period. Sometimes if the provider you are trying to credential, has a service that is unique or within a geographical area where that service is not offered a closed panel will make an exception. The next step is usually getting the application in. Some carriers will allow you to see their fee schedules first, but some won't until you get your credentialing packet or send in a letter of intent. Then it's a waiting game.. usually not so long for the open networks.
There is another part to credentialing which involves legal contract and fee schedule negotiations, but it's much too much to go into in a post and really should NOT be attempted unless you have experience. I wish i could refer you to books/courses but since it's an ever changing industry it wouldn't help. Like I said, most carriers have their own way of doing things. There is a standard uniform credentialing site at
http://www.caqh.org/ucd.php but I've always done my credentialing on my own, it's just cheaper and I can better devote my time to the details that way.