How do you prepare phosphate buffer solution of pH 8?

See the Web Links to the left of this answer.
I especially like the Smith.edu link -- it has complete and very useful description of how to prepare a buffer.

Use the Henderson-Hasselbach equation:pH = pKa + log [A-]/[HA]

where HA is the protonated form of the weak acid, A- is the salt (dissociated acid, or in other Words, its conjugate base), and the pKa is the strength of the acid.

What this says is that the pH that you want your buffer to be depends on two things:
-- the pKa of the weak acid you are using (see reference tables under the Web Links to the left)
-- and the RATIO of the concentration of the acid and the salt that you add to the solution.

The pH of the buffer does not depend on the actual concentration of the buffer, but on the ratio of the two parts.

The buffer capacity depends on two things -- how close to the pKa the pH of the buffer actually is (it should be within 1-2 pH units), and what the total concentration of the buffer is.

For instance if you have 0.001 M acetic acid and 0.001 M sodium acetate, the resulting buffer will have the exact same pH as a buffer made with 0.1 M acetic acid and 0.1 M sodium acetate (because the ratio is 1 to 1, the pH = pKa = 4.76). However, the 0.1 M buffer will have a much larger buffer capacity, and will much better resist changes in pH upon the addition of a strong acid or base.