Side Hustle and Best Practices

I’ve recently started a new venture aside from my primary job. 
Please, give more information on tax deductions based on expenses, inventory, and office space in home. How do you calculate office space for write off?
If you use your cell phone for a large portion of your business can you write off your entire bill or just a percentage of it. Same with internet?

Taxes based on profit/loss? Are you taxed based on profit or total sales? 

Advantages of creating an LLC for tax purposes?