Setting the amount you are willing to lose per trade is subjective. Generally, risk levels are set at between 1% and 5% of your trading accounts total balance. This means at a risk level of 5%, you can place 20 losing trades before you lose all your funds. If you find that you often lose 100% of your funds, you may wish to back track on your strategy.
Say for example, you deposited an initial amount of $1,000. To risk 2% per trade would be to set a stop losswhich will close the trade for you should a single trade lose $20 ($1,000 x 2% = $20).
Make sure that you manage your risk, as this is one of the pivotal aspects in long-term trading success.