The requirements to get on a panel are set by the insurance providers, and they can vary greatly. Asking about specific insurance plans and reimbursements is best done at a local level, though it is even more important to understand the broader implications of taking insurance, the different types, etc.
For general private practice information, you should look at joining the APA division for private practice, as they have a lot of resources. There are also seminars put on by the APA about private practice (probably sponsored by the division), and if you are at a point to pratice it may be worth going....I believe it is ~$250.
There are also a couple of books you should look into about building a private practice, which should have chapters on that. Walfish & Barnett wrote a book, "Financial Success in Mental Health Practice: Essential Tools and Strategies for Practioners". I haven't read it yet, but it came up a number of times on a list serv I read. The other is "The Paper Office" by Ed Zuckerman, which seems to be very well regarded.