Is this just a suspicion or gossip that you have been given. First try and see that there is truth in this issue of extra-marital affair. There are many private investigators, detectives, post matrimonial detectives who deal with proving or getting evidence of the same. This is very important before you come out in the open to accuse your spouse of having an affair. Under the Hindu Marriage Act, section 13(1)(i) adultery is one of the reasons to applying for divorce. Please answer truthfully to yourself, have you had or are you having a similar affair about which your wife would be or can be aware of?
Once you are convinced beyond doubt about this issue, then think it out keeping in mind the child, what does your family feel about this whole issue. Your options would be to have a discussion with your wife and in the interest of the child, go for counselling and get your marriage back on track. The other option is, if there is no way this relationship is going to end, then to apply for divorce on the grounds of extra-martial affair.
You can approach a good lawyer who specializes in marital cases to guide you further. Before you apply for a divorce, please give the below points some consideration.
1.The child born to you both, who would take care of it, the education, the expenses etc.
2. Money to be given to your wife and child for supporting themselves.
3.The joint properties and investments made by you both after marriage.
4. Any chance of a case being filed by your wife/her family for your property at a later date.
5. Any joint loans, EMI or annual payments to be made, how to go about it after separation.