First, before anything, you want to make sure that your employer really is breaking the law. People often wrongly assume that the law entitles them to things that aren't actually enshrined in law—such as fair treatment, paid vacation days, or a warning before being fired. So first make sure that you really are facing a legal violation.
If you are, you might assume that your first step should be to talk to a lawyer and file a lawsuit. But much of the time that isn't necessary at all, and jumping straight to a lawsuit—while certainly your prerogative—can unnecessarily poison your work environment.
Instead, a better first step is often to simply talk to your employer. Start from the assumption that they don't realize that there's a legal problem, and that you are courteously bringing it to their attention. It's often worth taking this approach even if you're pretty sure that your employers know their actions are illegal and just don't care. This stance will usually get you a better outcome than making it clear that you think your managers are flagrant law-breakers.
For instance, if your boss is requiring you to work unpaid overtime when you're non-exempt (the government category that determines whether you must be paid overtime or not), try working it out with your manager directly. Say something like, "We're actually required by federal law to pay overtime to people in my job category. I can work the overtime if you want me to, but the company is required to pay for it."
Or, if you're devoutly religious and your boss is requiring you to work on the Jewish holy day of Yom Kippur, when plenty of non-religious employees are available to cover that shift, you could say, "Yom Kippur is a religious holiday for me. I've spoken to several employees who don't feel their religious practice requires them to observe it. Because we're required by federal law to accommodate employees' religious practices, could we schedule them that day instead of me?"
Note that the tone here is collaborative, not adversarial. You're even saying "we" rather than "you" in talking about the company's obligations. And that's because your tone should be that you're looking out for the company's best interest, not making a legal threat—the same tone you'd use if you were advising your boss on another employee's request. There's no overt threat of legal action.
The reason for that is that your goal here is not just to assert your legal rights but also to keep a good relationship with your employer. It is possible to do both, but that's far less likely to happen if you wield the law like a weapon. Fair or not, the reality is that few relationships are unaffected when legal threats are made. You still have the option of taking legal action if it comes to that—but you're far more likely to get a good outcome by starting out this way.
Now, what if you talk to your employer and point out the law, but nothing changes? At that point, you have a decision to make about how far you want to push the issue. The law might give you a remedy, but realistically, it's also probably going to make your working environment difficult, and might even make it harder for you to get hired in the future if employers worry you're litigious. If you weigh those factors and decide to proceed, then your next logical step is to either talk with a lawyer or—depending on the law in play—your state's labor department.
But before you try that, try a straightforward conversation. It might be all you need to solve the problem with all parties happy.
Alison Green writes the popular Ask a Manager blog, where she dispenses advice on career, job search, and management issues. She's also the co-author of Managing to Change the World: The Nonprofit Manager's Guide to Getting Results, and former chief of staff of a successful nonprofit organization, where she oversaw day-to-day staff management, hiring, firing, and employee development.