If you are under age 18, there are some circumstances where a limited amount of investment income can be reported on a parent's return instead of a return in your name. This is called the "kiddie tax" and is fairly complicated so I'm not going to go into it. If you have investment income and want to discuss this, you and your parents should consult a tax advisor.
Sometimes the kiddie tax is not an option, and even when it is, a person always has the option of reporting investment income in their own name, so I will stick with that for now.
Any time you sell anything for more than you paid for it, you have taxable income. That's a general rule. This applies to eBay, garage sales, stocks, comic books and other collectibles, and virtual currency. It's considered investment income, or "unearned" income. If the asset was held more than one year, it is considered a "capital asset", so the income from selling it is considered "capital gains."
You have to keep accurate records of the items you sell, so you can prove if audited, what the original price was and the amount of taxable income or loss. (There is a recent tax court case involving a person who made $25,000 of gross sales on eBay in one year. She claimed everything she sold was sold for less than she paid for it, so she had no taxable profit. The tax court doubted that she was really only selling used personal items for less than she paid, and because she kept no records and had no proof of the items she sold, the tax court ruled that all her gross proceeds were taxable income, and assessed tax, interest, and a penalty for preparing a deliberately inaccurate tax return.)
As applied to virtual currency, buying the currency is treated as an investment. You buy some number of units of bitcoin, ethereum, etc. with US dollars. This by itself does not affect your taxes or need to file a tax return.
When you sell, you have a gain if you sell for more than you paid, and you have a loss if you sell for less than you paid. Gains are taxable income and losses are deductible and can offset other gains. How the gain is taxed depends on how long you held the asset before you sold it (one year or less; or more than one year.)
So you need to keep track of each unit you buy, with the date and price, and keep track of each unit sold, with the date and price, so you can calculate your gain or loss.
As long as you are a dependent of your parents, you must file a tax return if your investment profits in any tax year are more than $1050, or your total income from investments plus working is more than $6350. If you are not a dependent of your parents, you must file a tax return if your total income is more than $10,400.
