Choosing an online broker is easier then buying shoes.
Each of the online brokers (Scottrade, TD Ameritrade, Fidelity, etc) provide everything you need to buy and sell stocks through them. They are the vehicle that you will use, once you know what stock it is you want to buy. One major item that used to separate them was the price you had to pay to buy and sell a stock through them, but now even that has pretty much evened out. As I write this, Scottrade is $7 a trade which I believe is the lowest by a dollar or two. Once you select the online broker that you will use, you simply need to log on to the site and set up your account. Once it is funded, you are ready to start trading.