If you are literally just getting started, the services offered by most major stockbrokers (information here) as a part of their trading account services will be a good place to start (and free). Firms such as Trade King, eTrade, Charles Schwab and Ameritrade provide a range of online tools. These will give you a feel for how portfolio management software works without having to pay extra to learn. However, these services typically offer no advice (known as execution only), which means that a separate service will be required for information analysis.

Finally, keep in mind that if trading on margin—which means you're borrowing your investment funds from a brokerage firm (and bear in mind that margin requirements for day trading are high)—you're far more vulnerable to sharp price movements. Margin helps to amplify the trading results not just of profits, but of losses as well if a trade goes against you. Therefore, using stop losses is crucial when day trading on margin.
There are two types of trading platforms: prop platforms and commercial platforms. As their name indicates, commercial platforms are targeted at day traders and retail investors. They are characterized by ease-of-use and an assortment of helpful features, such as news feeds and charts, for investor education and research. Prop platforms, on the other hand, are customized platforms developed by large brokerages to suit their specific requirements and trading style.
One of the first decisions you’ll have to make is deciding what you want to trade. Every market is different, bringing with them their own benefits and drawbacks. You need at least $25,000 to start investing in the stock market for example, whereas the forex market requires the least amount of capital. You could start day trading with just $500 in your account.
For online platforms, websites give the user options and availability of the stock he wishes for, across all the platforms available. If he places an order, it gets stored in a database. After the confirmation of the trading account and payments, the order gets forwarded and the user gets his stock, in the form of money, transferred to his trading account.