Generally you have to hold a bachelors degree or higher and be considered an industry expert in a field. That can be relatively subjective in business fields. For example a field like Medical is simple, you have to have an MD and have practiced or done research in the specific subject matter of the case. For technology it is not so cut and dry.
For me personally being a CTO at three companies serves as my main qualifying credentials, even though the cases I take are deeply technical cases, cases you standard "I have never wrote code CTO's" would be over there head in. I have sold three travel related technology companies to some of the big players in that space which furthers my creditably in high volume online transaction systems, then there is obviously the specialist focus in travel technology. I have been a speaker at several conferences including IBM's Impact on the future of enterprise web and mobile technologies as well as rich internet applications.
Those are the markets I tend to stick to and it is easy to prove my credentials in those segments. While my list of credentials may look impressive it is by no means the bar that you would have to meet to be considered an expert. Competency, some business sense and networking is all that one really needs to get into the market.
After a few cases you are established as an expert witness and those become your credentials. As with anything else start small, take the cases the big fish are passing up because they only need an expert opinion of facts paper written or some other "lesser work" done. Those papers while not the same a expert testimony (many cases don't require expert testimony, just paperwork), do serve as creditably that you understand the legal process and understand your role. Obviously case wins are important to your reputation but not as much as they are to the lawyers.
I kind of fell into it, one of my companies was sued by Hotels.com, as well there was a lot of pressure being put on various governmental agencies to investigate our company for anti-trust violations. I served as expert witness in defense of my company. We settled out of court with Hotels.com which included the sale of the company to Hotels.com (it's how they negotiate) before the case drew to a conclusion. Anyway, a while later I received a call out of the blue from a legal group that was representing a client in a similar situation and was asked if I would serve as an expert witness, from there it just snowballed.
It is a pretty easy market to get into. Cold call, buy some lawyers lunch, give them a packet with your info and the types of expert information you can provide and ask them to network you in their network. Pay a commission if they hook you up with another firm.
Finally if you a serious about getting into the field of expert witness they you should get the book "A-Z Guide to Expert Witnessing" it is an expensive text but it is well worth the money. It has every piece of information that you will ever need to become a successful expert witness.