i am 15 and i play league and have trails, but their for union. What rules do i need to learn and what position should i go for if i play 2nd row/loose forward in league?

The positions in League are more interchangeable than in Union. Someone that played loose forward in League might be able to play second row, back row, or in the backs in Union. The ruck and maul laws will be the first thing you have to learn.