Well, first of all, get the TM43 and use it on a pokémon.
Secret Power has two effects, one in-battle and one out-of-battle. In battle, it's a Normal-type attack that has a side-effect...what side effect that is depends on the type of terrain you're standing on.
Out of battle, you can use Secret Power in three kinds of places. Look for large trees that look a little different from every other large tree. Look for square indents in rock surfaces, and look around Fortree for wide bushes. You can use Secret Power on any of them to create a Secret Base that you can decorate with dolls, mats, desks, and other fun items.