This is one of the questions a lot of new 3D printer owners ask. 

“What 3D modeling software can I use to create or edit my projects?”

Autodesk – 123D Design

For a 3D printer newbie I would suggest to start off with 123D Design from Autodesk.
It is a free CAD program that have a lot of features and probably enough strength for most newbies to do what they want to do. 

It can be downloaded from their website:

http://www.123dapp.com/design 

They also have a lot of tutorial videos to help you get started with the basics, like the one below on how to create your first objects.

http://www.123dapp.com/howto/design 

 You can even have a look at their gallery of available items and objects created by other users. You can download this as a starting point or something you want to print as is.

http://www.123dapp.com/Gallery/design