To enter WRC league you have to first finish the WRC 3 and WRC 2 seasons with a good ranking (1st is not required, I think) and choose a team of the next league at the end of the season. You can skip Junior WRC this time if you wish, there are no trophies tied to Junior WRC this time.
Unlike the previous WRC games, the trophy in WRC 9 does not pop immediately after achieving enough points in the ranking, it pops after finishing the final rally (Japan) of the season. You can still "skip" rallys you do not like as long as you have enough points at the end of the season. To do so just use the "quit rally" button in the rally camp menu. Careful, do not skip too many, I was able to skip the last 3 rallys and had just enough points to still win the WRC.
In the WRC season you will drive all 13 rallys of the game. If there are rallys in the game you still have not finished in a podium position, here is your chance for the respective trophies.