By the way I'm a Mechanical Design Engineer. And I was pretty poor at designing. And I feel this is pretty easy way to learn to design things.
My Mantra towards designing things is (if you're not a pro), start with pretty basic geometry and build from there. Since you'll be building things only virtually, you shouldn't be wasting any material or energy.
So, for the first tutorial let's move on with designing an ice cube. My choice of CAD software for accomplishing this would be Autodesk Fusion 360.
Being a simple person and also a math fanatic, I start with simple geometries.. something like below.