You must file a self-employed return if your net income after expenses is more than $400, that's not the threshold for making estimated payments.
You will owe a penalty for underpayment or late payment if you don't pay during the year, even if you pay in full when you file, unless you meet one of these three penalty exceptions:
1. the tax you owe when you file is less than $1000
2. the amount you paid into the system is more than 90% of your tax bill
3. the amount you paid into the system is more than 100% of last year's tax bill. (Your last year's tax is not your refund or tax owed, it is your tax liability. For example, if you had $2000 of W-2 withholding and got a $500 refund, your tax liability was $1500.)
If you have never worked before, and paid no tax last year, then you already have paid 100% of what you owed last year (because that was nothing). If you have paid tax in the past, then you probably need to make some estimated payments. Your tax return will calculate your actual tax owed from all your income, deductions and credits. If you over-pay your estimate, you will get the difference back as a tax refund. You can make estimated payments online at www.irs.gov/payments
