Autodesk Tinkercad is a free online CAD platform that makes it simple and quick to make 3D printable structures.
It runs in your browser so there is nothing to download before you get started.
Click Here to get Started
This Video shows you all the basic functions you need to get started in TinkerCad
The main way to sculpt objects in TinkerCad is to use basic primitives like cubes or spheres and group them together so that some connect and some take away from eachother.