Even though some companies say they offer "free online stock trading," no company can really give away a product for free, unless they are a non-profit whose goal is to teach bad stock traders how to do better trading. And since there are no such groups, you will have to do business with a company that charges you fees, either directly or indirectly.
Companies that offer "free online stock trading" usually give you free access to a members-only online stock trading site where you can use a variety of stock trading analysis tools. It will also usually give you access to dozens of free stock tips from different sources, including relevant newspaper clippings about publicly-owned companies.
If you choose a "free online stock trading" company that gives you a free membership, that site will probably make money by selling ad space or charging commissions on trades. This means that pop-ups, flashy ads, and biassed information could make it hard for you to trade stocks, or it could mean that you have to pay too much every time you make a trade.
On the other hand, some companies that offer "free online stock trading" charge fees to join but do not charge for trades. If you plan to trade a lot of small amounts of stock each month, you might want to go with one of these companies. They will charge you each month, but you won't have to pay fees when you trade. On the other hand, if you only want to make a few large trades, you might want to choose an online stock trading company that charges you per trade instead of per month.
Remember that there is no single best way to solve this problem for everyone. The best choice for one trader might not be the best choice for you. This is why it is important for you as a trader to look at each deal in terms of what it will give you.