I co signed a loan. the loam is now in default. how do I sue other person for balance

You have not given enough information for me to give you a good answer. If you "co-signed" on a loan, that suggests that you signed the loan to lend your credit to a borrower so that the creditor would give the borrower the loan. A co-signor is an accommodation maker, which is like a guarantor. If the borrower defaults on the loan, the creditor can come after the co-signor for repayment of the loan, and a creditor can collect from the co-signor directly. Your question, "how do I sue the other person for the balance" suggests to me that, perhaps, you are the lender/creditor or that the lender/creditor has already gotten payment from you, and now you are trying to be reimbursed from the borrower. The exact answer depends on which is the right scenario (or neither - maybe you see the writing on the wall and fear that the creditor will come after you). If the lender has not yet taken the borrower and/or you to court, you do not have any claim (yet) against the borrower. You should contact the borrower, however, and urge him/her to take care of the debt. If the lender has already sued, you would counter sue the borrower. If the lender has already collected from you, you would have to file a separate suit against the borrower. If you are the lender, then you need to send whatever notice is required by the loan documents, and then take the borrower to court if they don't pay. In any of those scenarios, I recommend getting the advice of an attorney and using an attorney to attempt to defend and/or pursue your claim.