I was nervous too, but when you beat the first one, and you've healed your Pokemon, just think, that was easy, then do that until you get to the 4th one and you'll be so confident, I'm sure you'll do well. That's what I did, and trust me, you should have a lot of money, I do, I could buy like 199 revives in one go, If I could, go around, face trainers, equip your main/strong Pokemon with an amulet coin, and then try and go back to the league, if you are feeling nervous. Then past the league, no more battles and there will be a place you can heal your Pokemon.