Hey there,
While it may sound simple, actual movement a lá Diablo is a rather complex thing within Unity. The keyword here is pathfinding, which is a system that has a look at you scene and computes a path from one position to another. But since that's rather complex and pretty much always involves plugins, let's just start with the basics.
To move from one position to another you take the target position and your current position. If you subtract your current position from your target position you get the so called direction value.
This direction value can be used in a number of actions, like the move by, translate (with every frame set to true) etc. Most actions ignore other gameObjects however. So to make sure your character does not walk through walls unity offers newcomers a premade "character controller" component. Once added to your gameObject, you can use the action "Simple move" to move in a direction. This also simulates gravity.
So now you can use "get position" to get your current position. Then you can use a "mouse pick" to determine the hit point of the current mouse position (= the point right under your mouse cursor). Then you can use the "vector3 operator" to calculate the direction vector. Then you can use that vector in the simple move action. If you turn every "everyFrame" to true, then this simple FSM should make yourcharacter follow your mouse cursor.
For actual pathfinding I highly recommend for you to get confortable with playmaker first.I'm writing an implementation for Aron's Astar Pathfinding at the moment, but it will still take some time and may just be rather complex.