Help me with my writing?

Eyy, soo yeah. I was wondering if, like, someone could please help me out a little bit. So, I've been told that I am a good author, for a thirteen year old, but my one problem is that I'm hecking dense and I always give away my plot twists. For instance, in a fanfic I was writing, I was trying to make the main character's best friend turn out to be an assassin who would in the end, try to kill the main character, but the readers figured it out looong before I wanted them to. I tried to drop small hints that no one would think much of, so that when the twist took place, they'd look back and realize what a significance those small things had. The thing is I aLwAyS mAkE iT tOo ObViOuS.

How do I fix that, lol.
Help me with my writing?
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