When you beat the game, you will unlock characters. The characters are Gudmund, giovanni, emma, kojo, and otto.
On some tracks, you have to jump over stuff to access shortcuts. To ensure that you do this without falling, Step 1: fill up your boost meter by drifting (press B while turning). 2: Press R to jump when you hit the edge of the jump (about the same time you shake the Wii Wheel on Mario Kart Wii). For extra long jumps (like the one in Rustbucket Stadium), use a full boost and jump at the same time. I hope these 2 tips help you reach some shortcuts!
To get a fast start, as soon as a race starts, hold the A button when the light is counting down.