In order to use Lego Mindstorms EV3 with Scratch you must install and run Scratch Link, turn on bluetooth in your computer and in EV3 Brick and load EV3 extension in Scrach.
Using Lego EV3 with scratch is quite limited, but sufficient to do simple exercises. We can use touch sensor, ultrasonic sensor and motor position. In color sensor we can only use brightness. We can't move the motor by an angle. Only per seconds.
In this video I've tried to show how to install the required stuff and how to use to motor and sensors.