I would ask to see a breakdown. There can be a whole long list of bits of your salary withheld for a variety of reasons. If you want to know what you'll be taking home, you need to know the 'kathara' earnings, not the 'meikta' which is what you've been told. I would ask for an 'analytiki misthodosia' to see where your salary will be going. You'll have income tax withholding, insurance, pension, and potentially other withholding as well (solidarity tax, etc). Before you can rent/buy a place to live you'll need to know your 'kathara' earnings. Any employer will understand that.
It depends on the employer and type of job so I can't answer that, but some of them are what I mentioned just before.
ETA: Welcome to the forum! I hope you will stick around