baby first food

So today was the day when i thought of starting oatmeal to my LO. I prepared it according to the instructions given on the box but my LO wasnt able to eat much. She started screaming because she was hungry and this was taking a lot of time. 1. so can you help me how much should I prepare considering this would be her initial food.? 2. what should be the ratio of oatmeal to formula milk? 3. My LO is having a hard time swallowing so any tips for that?