How much CSA should I be paying?

I’m 18 years old with a 3 month old little girl but also a 1 month old little boy both with different mothers I am in a relationship with my little boys mother we live in a 2 bedroom house so have rent and bills to pay for , the mother of my little girl has only just started to let me see her properly and is demanding money even tho I never get to see her.

I just wondered how much csa I would pay for her monthly as I have an income of approximately £1,300 a month but I of course have lots of bills going out every month also which I’m not sure if that is taken into consideration when it comes to how much I would have to give the mother?