This part is all up to you. There is no "line crossing," "arrow appearing" or "a small voice telling you to buy now!" You have to understand a little bit about how the price action works before you decide on your entry. Using our example, the Volume indicator shot up drastically meaning that traders are getting in on the action and thus driving the price upwards!
Assess how much capital you're willing to risk on each trade. Many successful day traders risk less than 1% to 2% of their account per trade. If you have a $40,000 trading account and are willing to risk 0.5% of your capital on each trade, your maximum loss per trade is $200 (0.005 x $40,000). Set aside a surplus amount of funds you can trade with and you're prepared to lose. Remember, it may or may not happen.