It's not clear from your post whether a class action would be possible here or whether you can best help yourself or your daughter right now by worrying about class actions. Speaking broadly and generally, for there to be a class action, a number of different people have to be experiencing a common problem, and your post doesn't say that anyone but you and your daughter are having basically the same or a similar problem with AT&T or a debt collection firm. Maybe other people are; but if they aren't, then a class action isn't the best way to solve your individual problem. (I've added "debt collection" and "debt settlement" tags to your post; maybe that will help you.)
Is there a debt collection agency involved or are the dunning letters all coming from AT&T itself? Are credit reporting agencies reporting erroneous adverse information that is hurting your or your daughter's credit score? You have important rights under federal and possibly also state law in such a situation. (I am not licensed in FL and can't offer advice or opinions specifically on your state's laws.)
But you describe a complicated and (to me at least) confusing situation. If you are eligible for pro bono legal help, a local or state bar association may find a volunteer attorney to help sort things out. Such help is available in my own home state, Maryland. Or there may be a state or local government office to contact for help with consumer issues, possibly a unit of the state attorney general's office. If you can afford to hire an attorney, I recommend a consultation. Or a legitimate consumer assistance organization (be careful; some are scams), perhaps run by your county government, may help.
One starts a class action lawsuit the same way one starts any other lawsuit: By filing a complaint with a Court.
Disclaimer: Any information provided through this forum is meant merely for informational purposes and is not legal/medical/tax advice. No attorney-client relationship exists as a result of this communication and to establish such a relationship, a potential client must enter into a written contract with the attorney. This attorney has not agreed to handle your case, nor will he do so until you have signed a written retainer agreement which outlines the obligations of both parties.
You may not be able to bring a class action against AT&T if your contract with AT&T contains an arbitration clause. And it probably does.
AT&T was the appellant in the landmark U.S. Supreme Court case, AT&T Mobility v. Concepcion, (2011), in which the Court ruled that the Federal Arbitration Act of 1925 preempts state laws that prohibit contracts from disallowing class-wide arbitration, such as the law previously upheld by the California Supreme Court in the case of Discover Bank v. Superior Court. By permitting contracts that exclude class action arbitration, the Court's decision makes it harder for consumers to file class action lawsuits.
If your contract has an arbitration provision like the one in AT&T v. Concepcion, you may be limited to bringing this dispute to arbitration, rather than suing in court.
Legal Information is Not Legal Advice My answer provides information about the law based on the limited information provided in the questions asked and is not intended to provide legal advice or opinions, and does not constitute an attorney-client relationship. The answer to the question is for educational and informational purposes only. The law differs in each jurisdiction and may be interpreted or applied differently depending on the jurisdiction or situation. Accordingly, I highly recommend that you consult with an attorney to discuss the details of your problem so you can get legal advice tailored to your particular circumstances.