If you have a monthly budget you follow, you will need to adjust it when you are having a new baby. You will need to estimate the amount you will spend the first year on having a baby. This included medical expenses, diapers, clothes, etc. You will also need to factor in childcare costs if needed.
Then you can divide this number by 12 to see what you need monthly. If it’s your first time having a baby, ask other parents what they spend monthly on a newborn to get an estimate of what money you might need. You might also need to factor in health care costs to add the child to your insurance plan.