A spouse may start an affair because something is lacking in the marriage or the spouse is not fully committed. You need to look at why the affair may have occurred and how it started. Don't doubt yourself within your marriage and your lack of worth, as it takes two to tango.
Don't blame yourself totally either or you will be an emotional wreck, your spouse is the one who cheated not you. Trust and love within the marriage is not something you can just pick up and start again when a spouse has had an affair, it takes time, patience and a lot of forgiveness. A lot of emotions and questions will be going round in your head just now so what you need to do is to take deep breaths and try to relax, don't let this overwhelm you completely. You will be in mourning, grieving and have a lot of anger about why it happened but you must look at repairing your marriage.
Communication with your spouse is the key, sit down and discuss what you both wish to do. Don't start accusations flying and asking questions as you will make your self even more miserable. All the questions can come later once you have strengthened your marriage again and you feel stronger within yourself because at the present time you will be feeling very vulnerable, not knowing where to turn to or why this has happened etc.
Discuss things in a calm and orderly way to make sense of what direction you both will go. Suggest a marriage counselor, they can make sure you take the correct steps to recovery.