After you have made a few trades, go to your account and find the statistics page. There you will find important information such as the number of winning trades you have made, the average win and the average loss. These three figures allow you to find the Kelly Ratio which, put quite simply, tells you how much of your capital you can afford to risk in a single trade. After all, you do want to trade as much of your capital as you can safely afford to in each and every trade you make!
Here is the formula: Kelly Ratio = p-(1-p)/[W/L]
where p= percentage of winning trades
W = average win
L = average loss
where p= percentage of winning trades
W = average win
L = average loss
So if you on average win only half the time, p=0.5, and if W = 4 and L = 2 then your Kelly Ratio would be 0.5-(1-0.5)/[0.4/0.2] which comes to 0.5 - 0.5/2 or 0.5 - 0.25 which equals 0.25. That tells you that you can afford to risk 25% of your capital on each trade without getting out of the game easily!