What is margin and how can i calculate how much i need?

Margin the amount of money required in your account in order to open a trade. Margin is a simple calculation based on the current market quote of the base currency vs USD, the volume requested, and the leverage level you have selected when opening your account. The dealing software will not allow you to open a position if you do not have sufficient free margin available.
Your free margin is indicated in the MT4 trading terminal.
To calculate the margin requirement required to open a trade, please use the following formula:
(Market Quote * Volume) / Leverage = $Margin required
eg.
You want to open 0.1 (10,000 base currency) lots of EUR/USD at the current market quote of 1.4177 and with a leverage level of 1:200.
(1.4177 * 10,000) / 200 =$70.89
As you can see from this example, you need at least $70.89 free margin at 1:200 leverage to open the trade. This shows you the power of the leverage offered by FXOpen. If you choose 1:1 leverage (or no leverage) you would need $14,177 free margin instead of $70.89 just to open this trade!