My children's father is trying to modify his child support payment it's asking me to respond with a financial declaration as well as a response to the petition. I have no clue on how to fill out these forms...

Do you have the forms which need to be filled out?

Many attorneys will do an initial consultation without charge. You might try that. If so, be sure to bring your financial records with you. If that option is not available, ask whoever does your tax returns to help with the financial declaration.

What State are you in ? Does this child support case involve the State Child Support Agency?

Most forms are (or should be( simple fill in the blank, or earnings, payments and so on.

Most support is based off custody and earnings. If asking for yours, it will deal with income of both parties.