It is easier to get started with ready-made file. Good project ideas and ready-made files can be found for example from these websites:
http://www.thingiverse.com/ (thousands of 3D files for printing)
https://all3dp.com/1/best-free-3d-printing-software-3d-printer-program/ (introduction to free 3D modelling software)
https://grabcad.com/library
http://makeabox.io/ (easy way to make a box)
http://3dprintingforbeginners.com
https://www.makerbot.com/img/educators-guidebook/MakerBot_Educators_Guidebook.pdf
https://grid.space/kiri (multipurpose slicing and visualization engine e.g. for 3D printing)
https://makezine.com/
https://www.instructables.com/
http://www.mynewsdesk.com/fi/an-cadsolutions/news/3d-tulostusmateriaaleista-tietoa-80005
https://www.youtube.com/watch?v=nfZE1QPM4Ns
https://www.epiloglaser.com/resources/sample-club.htm (samples with projects done with Epilog laser cutter)
https://www.troteclaser.com/en/applications/ (samples with projects done with Trotec laser cutter)
http://fi.rolanddg.eu/en-dk/Applications (samples with projects done with Roland table milling machine)