Important Things to Take into Consideration

I am an incoming High School Senior in the IB Program, and I was just wondering if anyone had advice on the Dos and Donts in college essays( as far as the admissions process goes).

What are some things one should include in their essays?
What are some things one should NOT include I their essays?
What are key features that the admissions individuals look for in ones essay?
As well as any other pointers, thanks !