just enter at the available sell price la straight away matched 

i gave u example of Ekovest stock below..

there is 847,000 available for you to buy at RM0.505..if you wanna buy 1,000,000 you will only matched at 847,000

the balance of 153,000 will then turn to left column become best buy at RM0.505

if someone sell his shares at RM0.505 for 200,000, you will be matched at all your remaining balance of 153,000 at RM0.505

but if no one sell his shares at RM0.505 price and price goes up, u will only get 847,000 unless u up your entry price to get the remaining balance 153,000 to match. please remember that your share price will be average out later. if u buy high you average price will be higher, if you buy low your average price will become lower.

and when market close, your order will be cancel. you have to purchase again tomorrow at RM0.505 sen if its available.

if you cannot brain this, i suggest just stay away from burning money in the stock markets. please attend some class or learn to trade out of the market using bursa marketplace something like that to learn first..