As you have just started, you are doing great! I'm willing to bet it would improve even without any comments from me, but since you asked.
Set up (that's when you have just grabbed the bar and you starting to lift):
Lean a bit forward, your shoulders can be a bit ahead of the bar and you can still maintain your back position during the first pull. Check your video around 25 sec, you are leaning too much back, which makes life difficult for you when you just about reach your knees. Your back is in good shape, but when you add weight make sure it stays that way (shoulders back, arched).
First pull (the bar rises to your knees):
You are doing good here. Keep calm, don't accelerate too fast and you'll be fine. When you fix your set up, this becomes more natural and it get's easier to go past your knees.
Now because of the earlier issues, the bar is pretty far from you and it makes it difficult for you to use your back and posterior properly. Your a rower so that's what you should use. Drive you hips forward and use your hands only to keep the bar close to your body. Try to do a couple from hang to get a good feel.
You are too strong for that weight, use more The bar comes really high, so you don't really have to pull yourself under it. Because the bar travels and ellipsoid before you catch it, you don't have as much balance as you could have. Learn how to freeze for a second on the catch-position and feel how your balance is. It should be solid at that point.
And when you lower the bar, keep it close to your body to avoid jolting your back. Now you kinda bounce it.