Difficult decisions....

How do you know when your ready to have a baby? Every fibre of my being tells me to have a child, but I'm so scared of being a bad mum. I'm 21 and after my next endo surgery I can use a donor to try for a baby. But I don't want to let that child down. How do you know if your mature enough? Or going to be able to give your child the emotional support they need going through their life? I want to do this so badly, but I'm petrified of not being good enough!