Artist-First Time

So confused:

1. I'm filing taxes for my art income for the first time. In trying to figure out my starting inventory - is it ok to say $0.   

2. To compute the cost of this year's inventory, some of my supplies for this tax year were purchased in prior years (e.g. paper, framing supplies, paint etc).  Do I just take a loss on those? Only compute cost based on what was purchased this year to create those works?

3. Also I mistakenly submitted my return without really understanding the prompts. So now I'm trying to amend my return to correct costs/inventory/expenses. Am I in trouble or ok if I amend before Apr 15th?

 

Any tips for a first-timer would be appreciated.