To build your very own secret base, you will need to talk to the person on Route 111 who is standing next to a big tree. He will give you TM43 - Secret Power. After teaching this to a Pokemon, scout around the world to find a location that you like. You can use Secret Power on indents in walls, by trees that have vines, or by grass bushes. You can change the location of your secret base at any time, but you may only have one secret base at any one time.