The basic use of any camera is almost the same no matter if photography or video. I wouldn´t mix up video with photo though in this thread.
First, you need the correct exposure (brightness of your clips). The factors that have an influence on the brightness are shutter speed, aperture and ISO (and of course the brightness of your scene). You can either use full auto mode, Aperture Priority or Shutter Priority. In Aperture Priority you set the aperture, and the camera tries to get a well-balanced exposure changing the other factors that determine the brightness of an image (Shutter Speed and ISO). In Shutter Priority, you set the shutter speed and the camera takes care for aperture and ISO. What you have to understand are the effects that aperture and shutter speed have on your image (video).
In video shutter speed isn´t all that important. Usuall,y you set it to 1/50sec (for Pal videos or 1/60sec for NTSC videos - pal is 25 frames per second recording, NTSC is 30 frames per second recording) to get a filmic look (smooth motion rather than the staccato effect you get from several action movies like "300").
That leaves us with the correct aperture. Before I explain too much - let me just link to one of my youtube videos:
(the effects of aperture) this and the two that follow should give you an idea how the aperture changes the look of your video. BUT: if you want that shallow depth I´m talking about, you will have a hard time with the a7sII´s focus - it won´t keep up.
So, you now know you need 1/50sec shutter speed and you know what aperture you want. If you set the ISO to auto, the camera will usually do a pretty good job exposing your clip. BUT: if you are out in the bright sunlight, you set your shutter speed to 1/50sec and you want your aperture at f1.8 - you will get a totally overexposed clip. Since there is no other way to darken the clip (ISO minimum is 100 for this camera, you can´t go lower), filmmakers use ND filters which are essentially sunglasses for your camera. There are different ND filters available that block more or less light.
The next important thing is focus. Up until recently, most serious videographers used manual focus because auto focus wasn´t reliable at all (except for some dedicated video cameras that have a very small image sensors and therefore very wide depth of focus) . There are several tools that help you with manual focussing like follow focus, etc. but that is something for the more experienced filmmaker. Your choice would be: either have a person that doesn´t move at all, set the focus once and leave it alone (but be aware that slight movements front - back can get your subject out of focus), or choose a camera that has decent auto focus for video (hence my recommendation in my first post and down below). Focus is critical for your clip, so is the exposure. If you get these two right, you are half way there.
However, let me once again recommend: if you can, do return the camera and get a a7 III or a7r II. Seriously!!! They have features like face detection autofocus and eye detection autofocus which is just amazing. While the a7s II was built for video use (up until recently, hardly anybody serious about video used autofocus, so this camera lacks a good autofocus) and for very low light photography (which hardly anybody ever uses). It does have features like slow motion (a7 III has that too), but you will probably never need that. So my advice is: change camera if you can. The other two aren´t much different to use, so the experience you gained during your weekend won´t be lost. I own all three of these cameras, so I am able to compare.
If that saves you 400₤, get a nice LED videolight to light indoor scenes with people.
I hope that helps.