A 22-year-old woman with Type 1 Diabetes presents to the Emergency Department feeling unwell, with nausea, vomiting, and abdominal pain. Her capillary blood glucose is 22 mmol/L and ketones are 3+ on urine dipstick. Her pH is 7.20 and bicarbonate is 10 mmol/L. What is the most important initial step in her management after securing IV access?