Simple 6-piece burr - For Starters!
thingiverse
This is probably the easiest classical 6-piece burr puzzle. It is a great one for beginners to learn the shape of the solution. After getting this one, maybe try this set: https://www.thingiverse.com/thing:4041348 It has a progression of increasingly difficult puzzles. This stl was generated with Enso (http://www.jeremyrayner.com/puzzles/enso/) from the attached .xmpuzzle file. enso will output OpenSCAD (https://www.openscad.org/) files which reference Aaron Siegel's excellent Puzzlecad library (https://www.thingiverse.com/thing:3198014) to do the final rendering as an STL. You definitely don't need the .xmpuzzle file to print it. It is a file that can be loaded by Burr Tools (http://burrtools.sourceforge.net/) which will allow you to explore this puzzle (and solve it). I recommend printing on a raft.
With this file you will be able to print Simple 6-piece burr - For Starters! with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Simple 6-piece burr - For Starters!.