The first step to using QGroundControl (QGC for short) is to install it. You can install it from the downloads section. Refer to the Bug Reporting section for dealing with any issues with the product.
The first screen you will see is this:
This is the Main Window for QGC. Not all options are visible since you are not connected to a vehicle. So the first step is to connect to your vehicle. If you click the Connect button you should see a dropdown of available connections to use. A vehicle connected via USB, or a 3DR Radio connected via USB will automatically show up in the dropdown. There is also a default option for connecting via UDP over Wi-Fi.
Once you connect you will be taken to the Setup View.