What Saves Us

First of all, the author of the poem tries to compare two situations in which the young soldier tries to make love to a woman before leaving for war and actually surviving that war.
We are wrapped around each other in the back of my father's car parked
The first two lines mean they are laying in the back of a car with their hands around themselves, possibly talking and cuddling. in the empty lot of the highschool of our failures, the sweat on her neck like oil.
'Empty lot' means it's probably after school when everyone's gone or perhaps in the hallway. 'of our failures' refers to their past. Enlisting was one of the choices upon graduation back in the day. Lastly, the girl has sweat coming down her body because they are getting intimate.
The next morning I would leave for the war and I thought I had something coming for that, I thought to myself that I would not die never having been inside her long body.
The author tells us that the soldier is fearful of never making love to this girl before he dies in a war. He refuses to accept the fact that it might happen. It's a now or never thing. 'never having been inside her' means not having sex with her. I pulled her skirt above her waist like an umbrella blown inside out by the storm.
Here, the author is describing the situation of how the soldier plans to go about it. Also, he compares her 'skirt' to an 'umbrella' in a form of a simile. If you pull a skirt up, it will stand like an umbrella.
I pulled her cotton panties up as high as we could stand. I was on fire. Heaven was in sight.
Here, the author continues to describe the situation of what is going on between the young soldier and a woman. He lets us know the material of her panties. By now, it is getting pretty serious because the young man starts to describe his feelings. 'On fire' means his body temperature has