Your mind WILL wander; that's part of what you will notice with your mindfulness: minds wander.
Begin by just sitting in this posture for a few minutes in this environment. If your attention wanders away, just gently bring it back to your body and the environment. The key word here is "gently." Your mind WILL wander; that's part of what you will notice with your mindfulness: minds wander. When you notice that yours has wandered, come back again to body and environment.