Tutoring Spanish and Portuguese could be an option for you.
It won't pay the bills in Poznan. I can speak from first hand experience - no-one is interested in young Portuguese/Spanish/Italian speakers "teaching" because they usually don't have much of an education, nor do they have any ability to actually teach. The going rate for such a person is as low as 20zl an hour - and everyone knows it. There's far too much of a supply on the market and little in the way of demand - Spanish is fashionable, but no-one is going to pay big bucks for it.
But I don't know how realistic is to find such job here without the language.
It is possible - there's one kebab shop in Poznan that seems to only employ foreigners.
There are other alternatives, such as finding work with a company that wants to move into the Spanish/Portuguese markets. But no-one is going to be interested in working with such a young person - especially in an ageist society like Poland.
How much of a hard time will I have finding a job that pays the rent plus all the basic stuff (food, bills, internet and all that)?
Very. The best hope you have is call centres, but it's hard hard work for low pay.