Why hasn't anyone linked to this?
Read that article, it explains about all of the basics you need to know.
A quick summery for once you know what you're doing:
theta - the angle of your vector
Polar to Cartesian Coordinents
x=magnitude * cosine(theta)
y=magnitude * sine(theta)
Cartesian to Polar Coordinents
magnitude_of_x_y = sqrt(x^2 + y^2)
theta = atan2(x,y) *it's atan2(y,x) in MATLAB and some other languages.