There are several ways to get a model to print
Thingiverse is a fantastic place to get free open source models.
Tinkercad is a tool we use in our programs as an introduction to 3D modeling. Tinkercad also has an online library of models you can pick from.
Fusion 360 is a far more powerful tool for 3D modeling, though it has a steeper learning curve.
To prepare a model for 3D printing, you need a tool called a slicer. Slicers take your model, and split them into layers (slices) of instructions.
For the Prusa's the slicer we use is Slic3r.
You have several tools to manipulate your model on the screen, including rotate, scale, split, and cut.
Step 1: Load your model.
This can by dragging it onto the build area, pressing the add button on the screen, or going to file --> Open STL/OBJ/AMF...
Once your model is loaded feel free to manipulate it how you want before step 2.
You should select your material, and quality settings now.
Step 2: Adding supports or rafts.
Some models will require supports or a raft to help them print. Anything with large overhangs, or a small bottom surface area can benefit from these features.
To add supports go to the print settings tab at the top of the window, and go to the support material tab.
Pick if you want supports everywhere, or only coming off the build plate. Slic3r will automatically add rafts under any supports touching the build plate.
Step 3: Prepare the printer.
To prepare the printer, first clean the bed with isopropyl alcohol swabs. This removes fingerprints and dust that would stop good adhesion.
Also check the filament roll to see if you have enough for your print.
Step 4: Send your file to a printer.
In Slic3r, press the slice button on the right hand side to generate a .GCODE preview of your model, then the save button to export your model. You will get a time and material estimate after you export it.
To print on the Prusa's upload your file to the SD card and insert it into the printer. Use the knob to navigate to your file.
Step 5: Print!!!