In 2014, insurance companies will send you a 1099. You attach this form to your 1040 individual return. This shows the IRS that you are covered as the ACA mandates.
If you are uninsured for that tax year, you must calculate the penalty for noncompliance on the 1040. Your tax for 2014 is the larger of $95 or 1% of income above the minimum amount required to file a tax return, not to exceed $285. That figure will increase in coming years.
The IRS can't charge interest on this tax, levy or seize property to collect the tax. They can, however, hold on to your tax refund, if any, to satisfy the debt.