How do you find diameter of a circle with the diameter 10?

You have answered this question for yourself; the answer is '10'

Do you mean to ask ; What is the circumference/area of a circle, with a diameter of '10'?.

If so , commit to memory ' elementary school maths.

A = pi r^2

C = 2 pi r or pi d .

From this 2r = d

Hence 2r = 10

r = 5

C = 10 pi = 31.41592.... units

A = pi 5^2 = 25 pi = 78.5398... units^2