This tutorial assumes you're already proficient with Blender. You can always consult the Blender resources page for links to good general-purpose Blender tutorials.
Modeling the Kart
Start with a cube or whatever your favorite starting object is in Blender. Your kart will need to be oriented with the front in the positive Y direction, so you might want to elongate your cube a little in the Y direction (green axis in Blender). When you hit the 1 key on your keyboard's numpad, you should see the back of your kart. The coordinate point (0,0,0) should be the center of the bottom of your kart. A few loop cuts on the cube should give you vertices to shape into a rough approximation of your kart. A mirror modifier can also be very useful.
Modeling the Character
I can give even less instruction on how to model an organic shape such as a person; however sculpting can be very helpful in getting a smooth look. The character should not be part of the same mesh as the kart.
Wheels are extremely easy, just model a cylinder into something you're satisfied with and mark the object as a "Wheel" type in the SuperTuxKart Object Properties panel.