Be advised that you are not able to use margin trades with every stock. You cannot use margin trades on initial public offerings, penny stocks, or other stocks that might be interpreted as being very risky.
The Federal Reserve Board determines which stocks can be traded on margin. The Board will decide how those stocks are to be used based on various factors, such as how much money is involved among other factors. The goal for the Board is to keep investors from spending too much money on risky stocks.