How can I get child support modified and get joint custody?

Good day to all. I just wanted some help. I went to court and when the support order was done, I was not ready with my paper work. I make $600 a week and have two other kids and they are taking $309 weekly which makes it hard for me to even support myself and my other kids. I asked court to modify it with the proof that I need like bills and lease because at this rate, I will be homeless and within the streets within months. And for the custody, I am going again because the mother won't let me see her. There is always an excuse. She has taken the baby out of the country without my consent just for the reason that she does not want me to see the baby. She also made her step father give me the affidavit for the court. She made him sign it and notarize it once the dealine for the court was over I have the proof when I gave her step father the papers for the court day. Now what can I do? What legal remedies can I do? My three kids are my life and I want to be able to support alll three of them. They are my life and my soul and they deserve to spend time with each other and their father. I am begging can somebody advise me or help me. Thank you.