I am picturing an inclined plane, sloping downward to the right. I am choosing a +x direction down the slope and a +y direction (which isn't actually needed here) perpendicular to and coming out of the plane. I'm going to call the angle of incline measured between the horizontal and the incline of the plane.
Stick an object of mass m on the plane and assume no friction. The object will begin to slide down the plane. Do a Free-Body Diagram. You'll have a normal force (N) in the +y direction, and a weight (w) straight down.
We need to break w into components in the x and y directions. (Specifically all we care about here is the component in the x direction.)
<-- Make sure you understand why this is sine and not cosine!
Since w = mg: .
Newton's 2nd in the x - direction: