Get legal advice before signing any agreement. Any part of the agreement that is about your child (child support, custody, parenting time, or visitation) can be changed later if you or the other parent can show that the facts have changed a lot. But there are complicated legal issues called "merger," "survival," and "incorporation" that make a difference to whether you can change the other parts of your agreement later on.
If the agreement says it is "merged and incorporated and does not survive as an independent contract," you can try to change it later if you can show the court that facts have changed.
But if the agreement says it "survives as an independent contract," the judge cannot change it later except in extreme circumstances. An example of an extreme circumstance is where you will end up on welfare if the judge does not increase what the surviving agreement says about alimony.
Ask for legal advice about "merger, survival and incorporation". Call your local legal services office to see if you can get free legal help. Or call a lawyer referral service to try to find a private lawyer to help you at a price you can afford.