Starting from what basis? If you are given a "symbol", i, such that , then the rest is easy. If not, then the usual construction of the complex numbers is:

Given the set of ordered pairs of real numbers, (x, y), define (x, y)+ (u, v)= (x+ u, y+ v) and (x, y)(u, v)= (xu- yv, xv+ yu) where x, y, u, v can be any real numbers.

Show that this set of pairs satisfies all of the properties of the complex numbers with (x, y)= (x, 0)+ (0, y)= x(1, 0)+ y(0, 1) and you identify (1, 0) with the real number 1 and (0, 1) with "i".