I think that what you mean is: evaluate the logarithm of 8 where the logarithm is taken to the base 8.
Definition: The logarithm of a number, to a particular base, is the power to which the base must be taken in order to give the given number.
i.e. Logb n = p where b^p = n
Since we have to find the log of the number 8, where the log is taken to the base of 8, then we need to find a power, p, such that 8 (the base) taken to the power, p, will equal the given number, 8.
i.e. we need to find p such that 8^p = 8
The answer is obviously p = 1