How do you find the number of neutrons and protons of an element?

The number of protons is just the atomic number and the neutrons is the weight minus the protons.

The elements atomic number is equal to the number of protons in the atom's nucleus.
The number of neutrons in the nucleus is equal to the atomic weight of the atom minus the atomic number.

For example, take the element Potassium (K),
Potassium's atomic number is 19, therefore it has 19 Protons in the nucleus (as well as 19 electrons in order to make a neutral atom).
The atomic weight of the most prevalent Potassium isotope is 39 amu....we know that 19 of that is due to the Protons, so subtracting 19 from 39 leaves us with the number of neutrons on the nucleus....20.
Very often you will see that the number of Protons and Neutrons in the nucleus is either the same, or very close to the same....this forms a stable atomic nucleus.  When the number of Protons and Neutrons are too different, the nucleus becomes unstable and tends to be radio active (for example, Uranium-235 has 92 Protons and 143 Neutrons).

As above but here's a chart.

Look at a periodic table. The atomic number (hydrogen 1, helium 2, etc) is the number of protons. The atomic weight (hydrogen 1, helium 4, etc) is the sum of the protons and neutrons. So, as you can see from the above examples, the number of neutrons in a helium atom would be 2.

Just like GOLDSPAN says:

Atomic Number = number of protons, also the number of electrons. (If the element is electrically balanced.)

Atomic Weight (the higher number on the periodic table) minus the Atomic Number = the number of neutrons.

Check you book, I'll bet the answer is there.

number of protons =the atomic number(ex. 3 for lithium)
neutrons =the weight(such as 1.00794 for the hydrogen)

One thing to be careful of: the periodic table gives the *average* atomic weight of the naturally occurring isotopes for the element. For example, natural chlorine is about half Cl-35 and half Cl-36 for an atomic weight of 35.45. Since Chlorine has atomic number 17, the Cl-35 atom has 18 neutrons and Cl-36 has 19.

Given an element's atomic number and mass number, you would be able to determine the number of protons and neutrons.
The atomic number of an element is basically the proton number of that element. The number of neutrons can be calculated as the mass number minus the atomic number, i.e. mass number minus number of protons.

In summary:
# of protons = atomic number
# of neutrons = mass number - atomic number