This depends in part on what you want to trade.
Despite all of the disadvantages (which you can see here), some people still want to trade stocks. Due to restrictive regulations, active trading in the largest stock market in the world (the United States) requires an account size of at least $25,000 by law. In reality, I feel that you need at least $30,000 to do so (preferably more to give you a cushion for losses).
To trade currencies, you can start with as little as a few hundred dollars. This has only been possible since the late nineties, when the foreign exchange market was opened to the
individual (retail) investor. For new traders that only have a few hundred dollars to risk in the markets, a forex account might be a good option. If you want to learn more about trading currencies, go to this section of my website.