Hey, for a no budget shoot, and a first(?) film, it's pretty good. That camera work on that first shot is really nice (as it is in the first shot of the second scene). I really liked the lamp at the end of the shot (and in other shots) - though it's framed a little awkwardly. The camerawork in general is really good. I loved the shot at the graveyard. Simple and unsettling - though your framing drifts a bit at the end.
It's quite flat colour-wise, and it's a bit too dark in some points - but I presume you were doing this without any sort of lighting (given there was a crew of two and no budget). You may be able to improve it a little more by tweaking the colours a bit in post. In future, get a cheap worklight or two (about $15-$30). You can use household items to control it. Bounce it off a wall to get an even fill. Shine it through a white sheet to diffuse it (experiment with different colour sheets). Use tin foil to control the shape. Etc. But you don't need to do that right now.
Screen is flickering in the background, but it's pretty forgiveable. This is the kind of thing that you only really figure out by doing. You can read all the advice you want, but unless you're specifically searching for stuff on screens flickering, it's not going to come up until you get practical experience. There are a number of ways around this - google it.
As you've already touched on, the sound isn't great. You won't make it much better, but I had to adjust my volume between the first and second scenes to understand the dialogue - so maybe just try to get a more steady mix.
Not a lot to say on the story. It's fine, entertaining enough, and makes sense. The dialogue isn't particularly naturalistic or believable - but you said it was improvised? One ting I didn't like was when you looked in the trunk of the boot - I couldn't see what we were meant to be looking at. The guy punching the bag then doing shots seemed a little weird, also. He didn't look like he'd been working out for long - and if they're in their own home, why would he bother keeping liqour in a flask? Their reaction when the guy with the gun comes into the yard isn't very believable either. They should be a lot more shocked/afraid/etc. Same as the reaction to death (the gun sound also didn't sound like it came from that gun). Ultimately, there wasn't a huge amount of build up or payoff with all the high stakes stuff going on. That's ok - keep practising your storytelling and writing, and you'll learn how to do that better.
I really liked the song at the end. Your music choices were pretty good throughout I thought - and you didn't do what a lot of people do early on which is drown out their dialogue with music and lyrics. The shot of the car badge then the car moving away seemed odd though. It seemed more like it should've been the first shot in that sequence (and all the shots in that sequence are nice, so I understand why you wanted to use them). Personally I would've had that shot, followed by a shot of him in the car (moving) with the sun roof coming off.
Editing is pretty good - apart from what I just mentioned. You said it was improvised, so I imagine that accounts for the long, singular takes of scenes - but I really like that style. If it was intentionally quite slowly cut, well done. It was engaging and strong enough to not feel "slow," but rather atmospheric. In some of the higher drama scenes, you could (since you don;t have vfx or practical effects or audio post professionals on your side) afford to quicken the editing up a lot to heighten that drama. The gun shots, hits and deaths aren't believable and don't feel that dramatic. You could make that really dramatic in the same style with better effects and sound - but that's unrealistic at this stage. Oh and I'm also not a fan of the many cuts to black and fades. That's mostly personal preference. But look for otherways to cut between scenes and location, and other ways to shot the passing of time.
Overall, I guess my feedback has been mostly negative. But it's actually pretty good. Your camerawork is pretty great. Your framing does drift, and it's a bit shaky at points, but as you gain more experience, it'll get better. You're certainly doing much more complex movement than i was. Again, I really liked the contrast of the moving shots and the shot in the graveyard, which emphasises how powerful a still camera can be. The main thing to work on would be storytelling, in my opinion. Work on your dialogue writing, and work on creating drama. Those are hard things to give advice on - and I'm not really a writer, so I'm not the right one to do that. But you can post excerpts on the screenwriting section of this forum and get some really good advice. After that would be sound. With a crew of two, your sound could be improved with the gear you've got by getting a bit closer with the mic, and using better boom technique. It'd also be improved by a better mix. You can probably find a student in your town who'd be willing to do it for free in future, and if s/he is passionate about sound, they can do it better. In the meantime, get a few worklights and play around with those for your next shoot
Well done on finishing your short - it's way better than my early work!