Beginners sometimes have trouble with thinking nothing. If you are just starting out, I suggest a focused meditation. It can either be your breathing or an object.
If you are going to focus on your breathing, as you breathe in visualize the air filling your lungs, then entering your blood, the blood carrying the air through your body, then back to the lungs for you to breathe out. Each breath of air bring in good air and each breath out carries away the toxins that were in your body.
If you focus on an object, close your eyes and hold the object in your hand. Feel the edges, curves, weight, elasticity, whatever special characteristics it has. Roll it around in your hands until you can create a perfect mental image of the object just from how it felt.