Discovering that your spouse has cheated on you or has been having an affair can present you with a crossroad from which many different roads branch. The number of paths, and the actions you can take from that point are almost limitless. But no matter how hurt, angry or resentful you are, the most important thing to remember is that you should NOT ACT OUT - not verbally, physically nor emotionally. Allowing your emotions to get the best of you can create significant problems for you, and can quickly result in your status changing from being the innocent party to being the "defendant". Assaulting your spouse, even something as minor as a "slap" or a thrown object, could result in your arrest and criminal charges. If you have children, this could present problems with regard to any hope you have of obtaining custody of the children or visitation with them. It is important to try and remain calm (a difficult thing to do, no doubt, but do your best).
You must first decide how the news will impact your marriage. Do you want to try and save your marriage or are you ready to pull the plug? Do you want to contact a marriage counselor or a divorce lawyer?
Assuming you are leaning towards divorce, you should gather as much documentation as you can regarding family finances and assets, make copies and keep them in a safe place. If you have a prenuptial agreement, now is the time to dig out a copy. Consult with a matrimonial and family law attorney as soon as you can.
Generally, it is not necessary for you to have evidence of the cheating, but this would be a question to ask your attorney during the consultation. In New York, infidelity is usually not an issue the Courts care about in a divorce proceeding, unless there is something connecting the affair to marital finances, or if there is a prenuptial agreement which specifically addresses this circumstance. If you expect the Court to punish your spouse for cheating, you will likely be disappointed. Discuss this with the attorney, ask all the questions you have regarding the issue, and find out whether or not the affair will make a difference in your specific case. Then move on to the other issues that are important to you: custody, visitation, child support, spousal support, division of marital assets, etc.
Use the consultation with the attorney to answer all your questions about the divorce process, and choose an attorney who makes you feel comfortable and confident.