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!
In addition to knowledge of basic trading procedures, day traders need to keep up on the latest stock market news and events that affect stocks—the Fed's interest rate plans, the economic outlook, etc. So do your homework. Make a wish list of stocks you'd like to trade and keep yourself informed about the selected companies and general markets. Scan business news and visit reliable financial websites.