The first thing to do is to select the source, prior to the recording mode. Then once the source has been selected properly, go to the Settings menu, and configure the recording options. Typically, you shouldn't modify anything since it works out of the boss in 99 % of the situations, but sometimes you have to modify those settings. Change the codec, resolution, processing, etc. Go back to the main window, make sure the source is selected properly, and the stream is working and press the Record button. You also need a capable PC of recording the content you use (video, desktop, gaming, etc).
I had issues with OBS Studio as well with the blank screen and this fixed it. Also, make sure that your graphical drivers are installed as well.