For starters you have to be good enough and fitter then most other sportsmen out there.
Next you need to be scouted (whicle playing for the first team of a high profile ametuer/semi-pro side)and (provided your young enough) offered an academy position at a club or franchise. After a season or two in an academy you may be offered a contract, although there's a strong possibility you'll end up only being able to play semi-pro in a lower division.
It's not easy and unless you play international rugby (which in your case will be possible being Polish, but on the flip side Poland isn't regarded as one of the 'prestigous' international sides) there's a high chance you won't earn enough to retire once your playing days are up, so you also need another trade to fall back on 15 years down the road.