We've been almost five months old, weight 7620 g. We had an appointmen…

We've been almost five months old, weight 7620 g. We had an appointment with a pediatrician, she said that the baby has elevated acidity and needs to start feeding with cereal. She said that at 5.1 we can start giving twice a day, in the morning and evening? How did you introduce the complementary food? Should we start with cereal or vegetables?