I think you're looking at this sideways. Instead of a plot, think about the characters driving that plot. For a harem game you need at least four: the Main/ Player Character (MC) and three love interests (LI). I'm assuming you're going 1 guy and 3 hot gals. Tell me about these 4 people. Let's start with superficial details on the love interests. We can go broad: bimbo, milf, girl-next-door, goth, or combine some types, say bimbo goth milf. Ideally we should be able to tell them apart, but still, think about hot chicks. Cause if you don't like them, how will the players?
Now, I don't want to explain character arcs right now; I don't think I'd do well and would come off presumptuous. But I want to mention our 3 LI all end up at one very specific place : She wants to fuck the MC, but she doesn't expect/ want exclusive claim to him. This is an interesting mindset, and the (hopefully different) paths that get them to that point is something you need to think about.
Two other points:
1st, I agree that you shouldn't worry too much about your basic skill as A) you get better with practice B) have you seen some of the shit out there?
2nd, there are plenty of books, articles,videos, etc. about writing. Read up on the basics if you feel that will help.
Hope that helps and best of luck.