The bid–ask spread is two sides of the same coin. The spread can be viewed as trading bonuses or costs according to different parties and different strategies. On one hand, traders who do NOT wish to queue their order, instead paying the market price, pay the spreads (costs). On the other hand, traders who wish to queue and wait for execution receive the spreads (bonuses). Some day trading strategies attempt to capture the spread as additional, or even the only, profits for successful trades.
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.