Newton's first law: An object in motion (or at rest) tends to stay in motion (or at rest) unless a force acts upon it.
Newton's second law: Force is equal to the product of an object's mass and its acceleration (F=ma).
The force that acts on a car when you hit the brakes is friction. The force of kinetic friction (fk) is given by the following equation:
fk=µkmg
The coefficient of kinetic friction (µk) is a constant value that indicates how much friction there is between two surfaces (in this case, rubber tires and asphalt). The term "m" is the mass of the car, and the term "g" is acceleration due to gravity (which is 9.8 m/s2)
When you have water between the two surfaces of rubber and pavement, you lower the coefficient of kinetic friction µk, and so the force of kinetic friction fk goes down. So you have less frictional force acting upon the car, which is why the car takes longer to stop (this is how it's related to Newton's first law).
The equation fk=µkmg is actually just a modified version of F=ma. fk is just a type of force (F). µk is a unitless constant and therefore does not change the dimensions of the equation. Since "a" is acceleration, and "g" is acceleration due to gravity, they're the same thing. Essentially, fk=µkmg and F=ma are the same equation. So you have a frictional force fk acting against the force of the car F (this is how it relates to Newton's second law).
Hope this helps. If you have any other questions, let me know.
*EDIT*
I modified Newton's first law, but it really doesn't matter because the car isn't at rest in your example. I just like being technical.
If you want to be really technical, the force of
kinetic friction initially comes from the brake pads and the tires are exerting a
static force of friction against the pavement. Once the brake pads grab, the tires start skidding against the pavement (at which point it goes from static to kinetic friction). But I think for the sake of your assignment, it's safe to drop these assumptions.
The picture is really simple. I'm assuming you have to draw a force diagram. Basically all you have to do is draw a car on pavement and show the only two forces acting on the car.