Case 1 - 300mm
Since you said that you are new to photography, I assume that you have a camera with crop sensor - this means that the image will appear to be more "zoomed in" than compared to full-frame cameras (which have a larger sensor)1
Shutter speed: As earth rotates, you can only expose for a limited amount of time before stars appear to be trailing. At 300mm APS-C (which is equivalent to 450mm FF), you are probably not able to have a shutter speed of over 1 second. The best is to try what works by making test images and zooming in on the photo to check for star trailing.
Aperture: C/2020 F3 ("Neowise") is, as most astrophotography objects, very faint. I would therefore suggest that you open the aperture as much as possible to let more light in (small f-number). If you find image sharpness (also look for the corners) to be not acceptable, stop down a bit (this means closing the aperture a bit).
ISO: Depending on your other settings, you will probably need a very high ISO. Try starting at around ISO 3200-6400 and if you feel that the image is to dark/bright, compensate ba increasing/lowering ISO. Note: This will cause a lot of noise. I will get to that later.
Case 2 - 50mm
Shutter speed: As the 50mm lens has a wider Field of View (FOV, basically you see more on the image), you can habe a longer shutter speed before star trailing happens. Try about 5 seconds.
Aperture: If your 50mm lens is a prime lens (i.e. fixed focal lenght, you cannot zoom), then it probably has a very wide Aperture such as f/1.8. Stop down to around f/2.8 at least to increase image sharpness.
ISO: Again, this depends on your other settings. An ISO of 1600 might be sufficient.
Foreground: (This is not directly technical) As the 50mm lens has a wider FOV, the comet will appear quite small. Try to include some sort of foreground (e.g. some trees, a horizon line etc.) to make the image more interesting.
General things - both Case 1 and 2
Shoot RAW: This is necessary. Astrophotography needs post-processing in order to make the images better.
Manual Mode: You will have to put your DSLR to Manual ("M") mode in order to change ISO, Aperture and shutter speed to your liking.
Focussing: Put your camera on a tripod and enable Liveview. Rotate the camera so that it points at a bright star that is visible on the screen or some distant light (e.g. a city). Put your camera to manual focus. Then, try to get the lights as small as possible by rotating the focus ring.
White balance: This does not matter too much since with shooting RAW, you can easily change WB in post without loosing quality. You can set it to Auto or, if you want a consistent WB, try choosing Daylight/Sunlight.
Remote release: You do not want shaky images. For long exposures, even touching the shutter is enough to cause camera shake. To avoid this, you can buy a cable release for around 20-30$. Alternatively, if your camera supports this, you can use an app for your smartphone to trigger the shutter without touching the camera.
Some other things
Image stacking: As I noted before, you are likely to have very noisy images. This cannot be avoided, especially with crop-sensor cameras. Many professional astrophotographers use star trackers which are devices that compensate for the earth's rotation and allow for long exposures without causing star trails, thus allowing for a lower ISO and less noise. However, star trackers are very expensive. This is where Image stacking is useful.
Basically, you take many images (usually around 15 up to 50 - depends on your subject). As the noise pattern is randomly, i.e. different in each image, you can average the noise out using stacking software. For Windows, you can use Sequator which is free.
This will work well for the 50mm lens. It might become a problem with the 300mm lens, since the comet will move out of the frame quite quickly (this is a reason why star trackers are needed for deep-sky astrophotography: You cannot take enough images on a static tripod for stacking before the subject moves out of frame). You can still try and see if it works.
Don't have to high expectations: Astrophotography is often considered one of the most challenging categorys of photography. I would suggest starting with the 50mm lens as you will probably be happier with the results.
I hope this somewhat helps!
1 This explanation is not entirely technically accurate or beautiful, but I thought it was the best way to explain it to a beginner.