Calculating numerically slope of a curve in a point

Hello. I want to find a way to calculate the slope in a curve (I want to write a function that works for every close curve like circle: x^2+y^2-1) in a point (x0,y0) without using 'sym' statement. I find diff and gradient statements but i'm new in matlab and i cant understand exactly how they work. I read that grad(F) give the gradient but F has to be a vector or matrix(?) . Please help. Thank you.