Plz help me frnds...

hi frnds...

i am planning to invest in Futures...
but, i heard that if the share price goes so low... some 30% of the money will be blocked from our demat account..
i am really confused with this stuff...
can anyone please temme the exact process ... why is that brokers tell us to put some money as reserve in our account ???....

and i am also looking for lot sizes and lot costs for NSE futures comnpany's...
in the link given in another post http://www.nseindia.com/content/fo/fo_mktlots.htm ... only lot size is given...
where can i get lot cost...

please frnds.. do temme if u know..

thanks in advance..

Shravan Koganti