Adultery is defined differently by state, but generally occurs when a spouse engages in sexual activity with another person outside the marriage. While adultery itself is actionable as a crime, pursuing an adultery action against your spouse will provide you the proof you may need to file a fault-based divorce. In order to accomplish this, you will need to contact your local police department or the county prosecutor's office to initiate an investigation.
Call your local prosecutor's office to file a complaint. The office will have walk-in hours, or you can set up a specific appointment. You will meet with a prosecutor who will review your evidence and make a decision as to whether the office will be willing to file charges, or what additional work will need to be completed. Note that many states may not be willing to commit scant resources to investigating a potential adultery case, and that many states have abolished, or are considering abolishing adultery as a crime.
Cooperate with law enforcement agents handling your case. If the police or the prosecutor's office begins investigating the case, they may need to interview you or receive copies of any evidence you have collected. If the matter goes to trial, you may be called as a witness.
File a compliant in civil court. There can be both a criminal and a civil aspect to an adultery charge. A person who consorts with a married individual can be sued for civil damages under a alienation of affection or criminal conversation claim. While this is an older cause of action, it is still common in some states such as North Carolina and Mississippi.