There are a few ways to go about this. One that is somewhat agnostic to what the equation is trying to represent (in this case, a circle) involves calculating the equation for the whole space, then plotting only an isoline of the target value.
[X,Y] = meshgrid(-3:.1:3,-3:.1:3);
Z = X.^2 + Y.^2;
If you know more about your function and can turn it around into a function of only one variable (e.g., sine and cosine of t), that is preferable in most cases.