Autodesk123D is a great way to start making your 3D models so you can print them out on a 3D printer.
There are libraries of ready-made blocks and objects and the software suite consists of programs for specific tasks:

Catch: take pictures of any object at all angles and ultimately crop to a 3D model


Sculpt: virtually 3D modeling clay


Creature: simply make your monster, alien, beast, demon, animal, etc...


Make: take your 3D models and turn them into do-it-yourself assembly projects


also there are programs such as Circuits for virtual breadboarding and design for your own circuitry. (imagine printing out your own motherboard in the future)

Sure the cost of a decent 3D printer and the consumables to print can be a grip, but the cost to use Autodesk 123D is free! (there are premiums) Ultimately in the long run you will make up for the costs if your 3D printer is put to use efficiently. The best part of it all is that you can let your imagination run wild, order a 3D print through the app to have your digital work become a tangible reality.