Win gold medals in 25 challenges
You unlock challenges as you progress through the game. By the time you reach Level 10 there will be a total of 50 challenges to choose from so I would suggest not spending time redoing the same challenge over and over as you get very little XP and it will just slow you down progressing the Levels. Just wait until you have reached Level 10 and then go Challenges you find the easiest.
Tip: The trick challenges can seem difficult towards to the end but just don't rush it. You need to the tricks in a slow controlled motion otherwise it will not register the right trick.