Scheming is an adjective that describes someone who is always doing sneaky things to make things happen, like your scheming friend who invites you to a family party because she secretly wants you to meet her adorable cousin.
A scheme is a plan of action that is usually secret or kept hidden. So something described as scheming has the quality of secretive planning. For example, you might offer to keep your mom company on a long drive to drop off a relative at the airport because on the way home, you plan to ask her to stop at the big shopping mall that's right off the highway. That's the essence of being scheming — you conceal your true intentions because you believe it will help you get what you want.