ECNs and exchanges are usually known to traders by a three- or four-letter designators, which identify the ECN or exchange on Level II stock screens. The first of these was Instinet (or "inet"), which was founded in 1969 as a way for major institutions to bypass the increasingly cumbersome and expensive NYSE, and to allow them to trade during hours when the exchanges were closed.[6] Early ECNs such as Instinet were very unfriendly to small investors, because they tended to give large institutions better prices than were available to the public. This resulted in a fragmented and sometimes illiquid market.
You need to understand basic day trading terminology & concepts to build your foundation. You can follow me on Youtube to get free education! Join the community of thousands of followers on YouTube and begin studying the free content we post on a daily basis. This is the beginning of your education. You need to study the markets, analyze charts, and learn the strategies professional traders are using every day.
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!

Many orders placed by investors and traders begin to execute as soon as the markets open in the morning, which contributes to price volatility. A seasoned player may be able to recognize patterns and pick appropriately to make profits. But for newbies, it may be better just to read the market without making any moves for the first 15 to 20 minutes. The middle hours are usually less volatile, and then movement begins to pick up again toward the closing bell. Though the rush hours offer opportunities, it’s safer for beginners to avoid them at first.