Nepalibhai,

When you buy, u need to put limit order not market value for stocks less $1, it might depend on the broker that u use. Personally, i don't buy  penny stocks but GNTA is recommendation from one friend who works in healthcare field.  Regarding margin, look at ur broker website for margin requirement, normally it depends on the price of the stock. But for penny stock, u can't buy them on margin. If you have other stocks in ur portfolio, check the balance that says "balance  to buy non-marginal securities" , u can only use that balance to buy these stocks.

PS: Buy these type of stocks with money that u can afford to loose.