Careful — minor spoilers ahead
To win the first tournament, you need to learn the team’s training strategy. Without it, you’ll just waste time training unnecessary skills. To do this, you need to find out the weak points of the opposing team, the Sandstorm Sirens. This is revealed through a storyline that starts with the quest "Meet your team at the Sports Center" (hint in the phone). At some point in this storyline, a quest will appear where you need to meet Celeste, get her drunk on wine, and learn about her team’s weaknesses. Even if you fail to complete this part correctly and Celeste doesn’t give you the information, you can still meet Samantha in the next quest and discuss the winning strategy — the tigress will give you some hints about what you should focus on training