What you need to do is to play well enough at the club level to be noticed by a professional side, and/or the various national selection committees. The player's age when this happens varies -- in nations with a strong rugby infrastructure (like England or New Zealand) players will be scouted at the U18 or even the U16 level. If they are felt to have sufficient promise, then they will be offered a spot at one of the "Rugby Acadamies", which are development programs managed by the various professional professional clubs. If the player is skilled enough, they will work their way up to the club's elite 15 squad, and if they continue to excel will by noticed by the national union. In places where there is no such infracture (like Canada or the USA), the national union encourages players who are sufficiently promising to play with one of the elite level clubs in that nation. If excel at that level then they might be promoted to the national program where they might have access to some athletic funding to help them with their living expenses as they train, as well as support from trainers and nutritionists. Some nations also have an agreement with a traditional rugby nation to help with development -- for example Canada has an agreement with the Welsh Union to place a number of Canadian prospects with Welsh development sides every year. And if they are sufficiently talented, they might even receive an offer from an European club to join one of the professional competitions.
