i want to share a flat with 1 person, in a medium-expensive area of the city
1500 PLN with utilities for one person in a two-bedroom apartment, all costs and utilities included. Living alone, a studio apartment will set you back about the same, and a one-bedroom apartment will cost about 2000 PLN or slightly more. Depends what you mean by "medium/expensive". If you mean expensive, you could pay double the prices I gave above.
i always cook at home, and maybe 2 times a month i go eating out (medium priced restaurant)
600 to 1000 PLN per month, depending on the diet you eat, including household items and toiletries.
i go out every weekend, but don't drink a lot (4 to 5 consumptions per night)
Anywhere from 30 PLN and up a night, if the only thing you have to pay for is beer. Really depends on what you mean by "going out".
All in all, it'll cost you about 3000 PLN to live a no frills life in Kraków. And that really means no frills, and no money left over at the end of the month, and no security blanket. The fun doesn't begin until you make 5000 PLN or more after taxes.