completely new

good afternoon everyone, im completely new to this, i dont want to sound like a complete idiot but how hard would it be to design a piston here if i have one sitting in front of me?

To be honest....
Can you draw what you want to achieve manually? People often think with a computer and CAD it's easier. It isn't. It's a a long and tedious way to learn it. You can try and have a play with simple things first. If you are not frustrated quickly you might march on.
LibreCAD is strictly 2-D only. For an engine design a 3-D software might be better.

Oh, I just realized you are talking of a piston only, not a complete piston engine. Well, that's doable.