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); % Generate domain.
Z = X.^2 + Y.^2; % Find function value everywhere in the domain.
contour(X,Y,Z,[4 4]) % Plot the isoline where the function value is 4.
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.
