Hi im set to pay 20% of my net income for child support if i open a business will i have to pay 20% of whatever net it makes

Your child support is based on your income. If you set up your business as an LLC or Corporation, the business will pay you a salary and that's what your child support would be based upon. However, if you set up the business as a D/B/A, you would basically pay child support based on the business income. (It's better to set up the business as its own entity, preferably an LLC taxed as an S-Corp, because you would save on taxes.)