What causes most contamination for beginners?

Most early contamination comes from three things: rushed or skipped sterile technique, substrates that were too wet or under-sterilised, and airborne spores from a dusty or busy room. Slowing down, cleaning surfaces with 70% isopropyl, working in still air, and getting moisture and sterilising times right removes the large majority of failures. Testing bought cultures on agar catches the rest.