A: Unfortunately, these kind of cases are increasingly common. Depending on your situation, the EFF may be able to recommend a lawyer near you.
Often times, the subpoena notice from your ISP will give you an opportunity to contest the release of your information in a limited amount of time. Finding an attorney to respond quickly to block the release can prevent further disclosures and put your case in a better position for later litigation. However, it can also be expensive so there is a trade-off based on how highly you value your privacy.
A particularly egregious example of how this litigation often plays out can be read about here: http://www.wired.com/threatlevel/2011/03/bittorrent/
Unfortunately, most attorneys would need more information on your specific situation, before they could advise you as to what action you should take in your case. Speaking with a knowledgeable IP attorney may allow you to resist the release of your information, but it depends on timing and the particular requirements of the subpoena.
A: In genera, the kind of suit you would file depends upon the kind of intellectual property that was infringed. For example, a suit for copyright infringement is federal subject matter and generally must be filed in federal court after the copyright has been registered. Patent infringement is also a federal subject and would be brought in federal court.
Trademarks are a little different, because they have both federal and state-level protection. An attorney can review your trade name or trademark and advise you on whether a federal or state court case would be best suited to the mark.
Finally, there are some forms of intellectual property (IP) such as trade secrets that are typically brought in state courts if both parties are in the same state. However, in any of these cases a legal situation called "diversity jurisdiction" may allow filing in federal court even for state-level subject matter.
Which avenue you choose to pursue depends upon the kind of IP that was stolen or infringed and the kinds of protection you have secured on that IP (such as a registered trademark or copyright).
Because each person or company's situation may be different, you should consult with an attorney for answers about your specific case.