If you just need to know how to detect input, You'll need to add a key to your input. Go to Project Settings -- Input Map. Then in the Action bar, you need to type in the name you want for the key, and press Add. Then, scroll down in the list, until you find the key you just added, and press the plus sign. then, add the key you want. Then, you can use the
is_action_pressed("name of your Input key") to check if your key is down. This may be enough, but depending on your code, you may also need
is_action_just_released("name of your Input key") too. If this isn't clear, or this isn't what you're having trouble with, please comment.