Hello everyone, I am new to the use of forums and the whole idea of posting and asking for questions online. I am a young inspiring game developer who’s currently a senior in high school taking a college game design class as well as a high school one, both are in unity. This is apart of the assignment to make a post, but I did have genuine questions on game design ideas. For one, I need help with the process of game development itself. I am in an amateur team of developers and there are times where the process needs refinement to get things done, but I am wondering the thought process I should have while developing games in order to help further increase productivity, as well as see how game development works in the industry itself. Another question I had was how I should start with a horror game I am developing in 3D. The main idea of it is relatively simple, a private investigator going into a town in the middle of nowhere to find his missing daughter who moved there relatively recently. I was planning on starting off with a cutscene explaining the game as you go into town and progress the game from there. How should I start developing this first part of the game, should I try working on the graphics or the mechanics of it first?
Always design the mechanics first. Quickly block out an idea, find what’s fun and then expand on that. Try to avoid the common pitfall of horror games of “find the key, open the drawer, get jumpscared, repeat”. Ignore the graphics for now, those will come later. Get something fun and playable as soon as possible.
try to find the look of your game, the style. is it topdown, first-person, cinematic, stylized etc.
same time need to have a story. a conflict and resolution / game mission objective.
It is also a question of scope. How much time do you have to develop something? In general, it’s a good idea to start with a fun mechanic, like whoft said. Only implement and test this mechanic and build on it if it’s really fun. Also try to determine what the focus of your game is. Horror can be many things. Unsettling music, frightening noises from different directions, the fear of being followed, of not knowing what’s around the next corner, or knowing… 
Don’t try to develop a game from start to finish. Game development is often an iterative process in which many things are tried out. So I wouldn’t recommend starting with a cutscene at the start of development. Find what makes your game an engaging horror game and build on that. I wish you good luck with your assignment!