where do I put the expense for farm work like cutting hay or bailing hay?

It depends. If your 19 year old does not own a farm, and is conducting this activity as a self employed person then it can be filed as a business and not a farm. All expenses ordinary, reasonable and necessary to earn the income is allowed to be used against the income.

• Reporting Self-Employment Business Income and Deductions
• How do I report income from self-employment?

For more information on farming you can review this publication: IRS Publication 225

• What is Schedule F: Profit or Loss from Farming
  • TurboTax Online: Select Income & Expenses > Scroll to Rentals, Royalties, and Farm > Select Farm Income and Farm Rental
  • TurboTax Desktop: Search (upper right) type farm > Click Add/Edit farming Info... link