Slopes

How would I go about adding slopes in stencyl? Like a hill.

Make the shape you want into a tile and add it in your game.

But how do I do the collision

But how do I do the collision

You can specify the collision shape (or create your own new one) right in the tileset editor.  Just click on the tile that you want to be a slope, and some options should display on the right side of the screen.