How much should I be paying my ex?

I am writing as a father, i am 22 and im a student in uni training to be a teacher and my ex girlfriend wants £100 a month from i earn from student loans 7,032 per year. The baby hasnt stayed with me as of yet, i only met him for the forst time this week after 3 months due to the conflict what went on. I dont work, i just study and study on placement in schools which is unpaid also. Now i was wondering what i should be giving her per month? If you could help please. Iknow she claims benefits but dont know what they are.