Where would I begin in repairing this bumper? It is pushed in and down on the driver side.

 

Take a light and crawl under the truck, look at where the bumper is bolted onto the frame.
See if the frame is bent when compared to the other side.

Your header panel is also broken and out of alignment and that part is not actually part of the bumper.
It's bolted on the front of the truck at various points in front of the rad and just above the side of the pumper.
The pumper has to be removed to take it off.

The fact that it's quite crooked and pulled away from the headlight may indicate that the sheet metal underneath is also bent.

Here's a link as to what is available and the header panel, as well as the pumper and its bits.

I would start by removing the four bolts on the pumper where it attaches to the end of frame rails to assess the damage.

Note too that the pumper can't actually be repaired because it's not worth the labour to straighten it, it's less expensive to buy a new one _ as well as the plastic bits that snap into it.
If you attempt to save the plastic parts, the tabs that snap into the bumper usually just brake off.

I replace my damaged pumper and got all the parts from the dealer, sort of expensive, but not too bad.
Note too that I'm in Canada, so even though there were lots of sellers that had inexpensive bumper parts in the US, the shipping to Canada became prohibitive.
There is nothing in Canada other then the dealers when it came to the pumper.

https://www.rockauto.com/en/catalog/...6,1416765,body