First, consider your motivation. Whistleblowing should be from a foundation of ethics, fairness, justice. Is blowing the whistle in the public interest? Could it help you or others? If you make a false allegation (a knowingly fabricated allegation) this is a form of misconduct for which you can be investigated. In some jurisdictions, depending on the type of false allegation, there could be criminal penalties.
Identify the perceived/actual violation. How? Explore relevant laws, Code(s) of Ethics, and policies that are applicable to the situation. Some forms of misconduct are legal but unethical (violating a Code of Ethics or policy).
Next, get some advice. Read about similar misconduct cases online (e.g., Google search; Tools & Resources). Discuss the situation with your partner/spouse as the negative impact of whistleblowing will extend beyond yourself. Get legal advice if you think you need it (try a free clinic if funds are limited or ask for a free consult). If you are having ethical distress, you might benefit from an ethics consultation. Ethicists can give advice regarding whistleblowing such as case analysis and potential solutions from an ethics lens, as well as guidance to resolve moral distress.
Gathering evidence? Be aware that covert audio or videotaping is illegal in some jurisdictions.
If you decide to be a whistleblower, use the proper reporting channel to file the allegation. Often, there is a provision to remain anonymous but not always. Sometimes, anonymity can be broken (accidentally, or otherwise).
Are there laws to protect whistleblowers? The following link contains a list of international whistleblowing laws: https://whistleblower.org/fast-facts/