You need to make sure if the Visa number has become available. For that, you need to see on the Visa bulletin. If the Visa number has become available for your spouse, your case will be transferred to National Visa Center. NVC will send you a letter explaining the next steps like 1. Pay for affidavit support - $88. Apply for affidavit support with supportive documents.
2. Pay $230 for visa processing.