If you're new to HIIT or treadmill workouts, start with a less intense routine.
For example, you could try a 30-second sprint at 6–7 mph, followed by a 1-minute walk at 3–4 mph, repeated 5–10 times.
My own personal HIIT session: 5 minute warmup at 3.5 setting incline of 6. Setting of 8 for 20 seconds. 40 seconds at 3.5. Repeat 8-10 times.