One question that comes up fairly often is: Where do you get your models? First for the newbies to 3D printing, the STL file is the most common file used to create 3D prints. Most 3D cad or solid modeling software create and can save this type of file.
You really have 2 choices using STL files. You can download models that someone else has created or you can learn to create your own models.
The first option is by far the easiest. A quick Google search will result an abundant supply of pre-made model files. My favorite is Thingiverse or Pinshape. 3Ders.org is a top 10 review for sites with STL files. A lot of the models are free and can easily be downloaded to your computer and load directly into 3D printing software such as Repetier or Cura. The disadvantage of this choice is that you cannot print a part unless you can find one.
The second choice is modeling your own parts. While this opens the door for seemingly endless creativity, there is a learning curve. As a design engineer I have used solid modeling software to create parts and specification drawings for many years. I learned on Pro Engineer (now called CREO). When I started BobsCNC I need software I could afford. Free is always good. I found a couple of free solutions that work good for me.
OpenSCad is a good choice for those who like computer programming. You type code and a model is created. If you are familiar with writing code this may be a good option. If you are not a programmer this can have a steep learning curve but a fun way to create models.
Design Spark Mechanical is a fantastic modeling software that is intuitive. It is a free version of SpaceClaim. The fundamental SpaceClaim tutorials can be viewed and within a few hours you will be able create models successfully. I have helped several folks that are brand new to modeling get started with Design Spark Mechanical and all have been successful in creating he parts they needed.
I liked the Design Spark Mechanical so well I purchased the SpaceClaim version. While SpaceClaim is not need for creating STL files fro 3D printing. It is the best solid overall solid modeling package I have ever used.