The trials are where you will start the game. Trial 1 is the Riding Trials and you will be tasked with completing the basic move set here. The board and character selection does not really matter for the trials so just pick what you like. Prior to starting the event the game will run you through each task and give you a diagram of how to do each of them. In Trial 1 you need to do the following in a single run:
"Face your feet sideways to go."
"Face your feet forward to stop."
"Lean forward to speed up."
"Tilt your body to the left and right to pass between the flags." You only need to make it through a couple of them.
"Raise your front foot and stomp to activate your item." First, you need to ride over one of the glowing lines to actually pick-up an item, then you can use it.