With online stock trading you're looking for short-term gains in what can be a volatile market. Doing you research when picking which stock to invest in, when to buy and when to sell, is extremely important.
If you're not an experienced trader, it can help to pick a trading platform that gives you the insights you need to make those decisions. But keeping yourself informed and up to date about how the market is performing will help you hone those skills over time.
That said, even the most experienced traders lose money from time to time, as there is always a risk when trading stocks.