First, it is a bad idea filing a lawsuit by yourself. A simple suit would be one thing, but a suit requiring third-party subpoenas and pseudonymous parties? This will require detailed understanding of civil procedure rules that you do not have.
That said, the way this is done is filing against a "John Doe" defendant, then moving for leave to take early discovery in the form of a third-party subpoena to the website that has the IP address. However, they might not hold that information forever.
Again, it's a bad idea to try this yourself. You should contact and attorney that *does* know how to do this.
I am an attorney, but I am not YOUR attorney. By providing free, generalized information, I am not entering into an attorney/client relationship with you, nor am I providing legal advice applicable to your particular needs.
I agree with Mr. Matesky's answer above - trying to navigate the legal system without an attorney in general is difficult, but you are talking about very complex procedural and substantive issues. If you are already represented by legal counsel, he/she should be competent to do so, even when it comes to the Internet legal issues. But if not, your attorney has an obligation to either get up to speed or associate with someone who does have the experience. You should have a discussion with your attorney about your concerns and then decide what direction to go. Best to you going forward.
I'm licensed to practice law in Michigan (www.shinnlegal.com). My response is provided only to educate the public about general issues that should be discussed with competent legal counsel in your state. Under no circumstances should you consider my response as a substitute for consulting an attorney in order to fully understand how the law may apply to your specific and unique circumstances.
As others have said here, this is not an easy suit for a "Pro Se" (someone representing themselves) to accomplish. Further, over 90% of Pro Se lawsuits are thrown out due to procedural matters (I am not talking about small claims court here.) Representing yourself in a complicated Internet suit is not a good idea.
Speak to your lawyer about your dissatisfaction. If you have already fired him, look for someone on AVVO who lists Internet law as a part of their practice.
Even if you don't want to hire a full time lawyer, you can at least hire a lawyer who will be your "Pro Se" advisor. They can help you write documents etc. and get you through the proceedural aspects of the law suit.
Finally, even doing a law suit as a pro se you are going to spend a lot of money which you are not likely to recoup. Perhaps you would be better off just trying to get the post taken down. A simpler and cheeper alternative to a law suit.
This post is provided for general informational purposes only and is not intended to be legal advice specific to you. This general information is not a substitute for the advice of an attorney in your jurisdiction. The attorney client relationship is not established by this post.