It depends on what you are filing and where the petition is going to be adjudicated. If you are going through consular processing, prior to the interview, the National Visa Center will send the petitioner instructions for payment of the affidavit of support fee and the immigrant visa fees and then require completion of various forms and submission of those forms and supporting documentation prior to the interview being set.
If you are going through the process within the US, you do not pay any additional fees other than the fees for the petitions you are submitting.
Please let me know if you have any additional questions or how I can be of further assistance to you in this matter.