Imaginary numbers and their polar form

Hey guys, for my Complex Analysis exam coming up, my lecturer has said that no calculators will be allowed. This is fine, but I've been having trouble switching between imaginary numbers' standard and polar forms. I can find the polar forms of numbers that have even denominators in their polar for (i.e. i = e^(pi/2)i), but I don't know how to find the polar form of numbers with odd numerators (i.e. -1/2 + (sqrt(3)/2)i = e^(2pi/3)i). If anyone could help me with this, or has an online resource that I can look at, I'd greatly appreciate it.