This deadlift begins with the barbell resting on the floor. Both the hips and knees are flexed to allow the hands to reach the barbell, and the back is slightly arched (never rounded) to protect the spine.
There are various hands and feet positions, so it's advised to practice with someone who has already mastered the barbell deadlift.
Inhale and hold the breath to create intra-abdominal pressure and lift the weight from the floor by extending the hips and straightening the knees. Assume a standing position, without locking the knees, before returning downward towards the floor.
The barbell must contact the floor before beginning your next rep. It's the "dead" weight that makes it a deadlift.
When to exhale is a personal choice. Some choose to complete the rep, then exhale. Others slowly exhale during the movement. Do what feels comfortable and safe to you.
Make it a great day!