I would definitley recommend seeing a tax professional when you are preparing to file your taxes, expecially when dealing with cryptocurrency. I am im not a tax professional, this is for educational purposes and is in no way advice for how you should be filing your taxes.
In the USA, any time you swap one token for a different token or cash a token out to fiat, it triggers a taxable event. If the asset was held for under 1 year, then it is considered short term capital gains. It will be totalled up with your other regular income and taxed in your specific income tax bracket for the year.
If the asset was held for longer than 1 year, it is considered long term capital gains and is taxed separately from your income in it’s own tax brackets, usually lower than your regular income tax bracket, but just depends on different factors. You can check out these sites for a better explanation and charts breaking down the different brackets:
To answer your first question, as far as I’m aware, there isn’t any lower limit on the amount of capital gains you are required to report, but I could be wrong about that. In other words, even if you only gained $5, you would still technically be required to report it. Also, I believe you can offset your reported amount of total capital gains with any losses you might’ve incurred up to a total amount of -$3000 per year.