Combat is different from standard Minecraft. In this universe you are an aspiring Pixelmon trainer. You selected your starter Pixelmon. In order to battle you must press the default 'R' key and throw your Pixelmon out in such a way that it makes contact with a wild Pixelmon or, if you wish to engage in a trainer battle, with a wandering trainer. 

Be mindful, trainers may come after you and wild Pixelmon may come after you as well.