My girlfriend recently told me she was pregnant..?

Recently, my girlfriend of 2 years told me she was pregnant. I'm overjoyed but I already have a 3 year old daughter from a previous relationship. Her mother died a few weeks after she was born, so she's been in my care ever since. So with a 3 year old daughter I don't know if I'm ready for another child, because I am only 21 years old. I really don't know what to do, can someone give me some advice please?