I would start by going through the Tutorial: http://tutorial.3dgamestudio.net/
This teaches a lot of basics, and even if you want to make a 2d game, lots of the things they explain there about 2d objects are also needed when you use/create/delete panels (which is the object that is used in 3dgs to draw images onto the screen).
Buttons are explained in the tutorial as well, try working with those.