Well the sad and simple way:

- Food: Go to the cheapest store that still has normal vegtables, cook for yourself whenever possible, don't buy overly expensive stuff like avokados or good cheese.

- Transportation: Bike/Foot

- Bakery/Concerts/Vacation/Cinema/Bars/mobile-internet etc: don't

- Accomodation: shared

- Other necessities: only buy when broken, google for 3 hours to find the cheapest thing that won't break all too easily or go 2nd hand. Save at least 1000 bucks in case something big like your laptop breaks.