A sketch's job is to help you communicate. To share the idea that is in your head.
So when you start wondering things like:
"What should my sketch show?"
"What fidelity should my sketch be?"
"Is a sketch what I need?
Your answer lies in understanding WHO your sketch is for.
WHO do you want to have a conversation with?
Your boss?
Your engineering team?
Your marketing team?
Your consumer?
Yourself?
Answer this question first and the other questions will start answering themselves.