This is one of the foundation lessons that everyone using Scratch needs. Conditionals are what gives your code the power to react and respond. If something happens (in your game, or in the real world through a sensor or user input), then a certain response is triggered. And you can learn all about them in a 30 minute exercise in your HQ.GirlsWhoCode account.
Or, start with your Girls Who Code account, and then go to the Scratch Tutorials link on the left hand column, near the bottom.
It helps if you can use two screens, like a tablet next to a laptop. But if you only have one, then open two browser tabs, and switch back and forth between the GWC tutorial page and your Scratch page.