How to avoid common mistakes first time entrepreneurs make?

In my experience, first you have to identify the 3 pillars of your business:

• Product / Service (or what ever you are creating)
  -websites, app, technology, services, products, & etc
  
• Operations (the things that keep productions & marketing in places)
  -Human resource, financials, communications & etc)
  
• Commercialization (the ways to get new & lasting revenue)
  -Sales, marketing, business development, branding, PR & etc)
  

The idea is, your job is to balance the 3 pillars for optimized outputs.
Put in real thoughts in these 3 pillars, to avoid as much risk and uncertainties as possible.
Don't risk too much, don't aim too low, find the balance yourself.

And let's not forget, the fuel to all 3 pillars is human.