If you buy ETFs from Amtrade in a standard brokerage account (not an IRA), you should know that they could result in taxable income. Any gains you make from selling an ETF will be taxed according to capital gains tax rules, and any dividends you receive will likely be taxable as well.
Of course, if you invest in ETFs through an IRA, you won't have to worry about capital gains or dividend taxes. In a traditional IRA, money in the account is only considered taxable income after it is withdrawn, while Roth IRA investments aren't taxable at all in most cases.
ETFs don’t have minimum investment requirements -- at least not in the same sense that mutual funds do. However, ETFs trade on a per-share basis, so unless your broker offers the ability to buy fractional shares of stock, you’ll need at least the current price of one share to get started.