To take a photo with Photobooth, push the red camera icon, this will initiate a countdown and will then take a picture.
You can also add cool effects to your pictures. To do this, click the "Effects" button and select the type of effect you would like to use.
Photobooth also allows you to take video. To do this, select the film reel icon. Click the film camera button and it will start recording video, to stop recording click the red film camera button again.