Before one can trade it is important to sign up with a trading platform. There are many trading platforms and based on trading style one should choose one appropriately. Remember trading cost can quickly add up and burn through your trading profits, so choose one which gives you the optimal cost. Here are some based on my experience
9.99 per trade i.e. when you buy or sell. Quite costly and is suitable for traders who are long term investors and choose value trading as their strategy. Buying frequency of 1ce or twice a month. Though ameritrade provides free features like Strategy Desk which is an excellent tool if you want to do rule based trading or charting and technical analysis. They also have online java version of charting tools. Also provides mobile cell trading interface.
Platform fees of 100 a month and a few dollars for data feeds. Stock buy or sell for a dollar. If you are serious about trading and planning on making at least a few dozen trades a month this is the best option. Also the emphasis of tradestation is on strategy trading i.e. you can program in a language called easy language using automated trading rules. The platform is superb. Also it provides a simple web based trading site beside the windows platform.
3 dollar a trade. If you are going to be trading passively i.e. on and off based on season and allowed time, this is the best option, because it is much cheaper than ameritrade, and you are not commuted to 100 bucks every month like tradestation. There are a few others like http://www.just2trade.com and http://www.zecco.com that also provide cheap trading. With these options you may want to look at other sources for technical analysis/charting tools.
So the trading platform should be chosen based on your style and how much you want to loose per month or per trade and the feature trade offs.