Blender is a notoriously complex 3D modeling tool, typically used for sculptural and cinematic work although it can also be used for solid modeling (for 3D Printing).
There are many tutorials on the internet for it and if you have little to no 3D modeling knowledge, it is best to start with a guide for beginners before trying more complex stuff.
This tutorial covers every step in the 3D modeling workflow from placing your first object to rendering a detailed object. The first video is below and the entire playlist is linked below.
With the full playlist here.
This tutorial covers the basics of using Blender to model something for 3D printing.
And a more comprehensive one.