First, in Tinkercad, (tinkercad.com) you can create the ramp's two main dimensions - height and length - with a Wedge from the Geometrics dropdown on the right. I want a long, gentle ramp, rather than a short, drastic incline
, so I'll start with 48" long by 9" high. For now, the Tinkercad build area is limited in size, so just scale it down to inches (1" = 1', or 1:12) I made the wedge's dimensions 48" long by 9" high and 30" wide (or4" x 3/4" x 2 1/2" at 1:12 scale).
Now you have a solid model of your ramp. You can take it to 123D Make by downloading from Tinkercad
Under the Design menu, click Download for 3D Printing and save the file as an STL on your computer.