Full Multivariate Polynomial function

hi I have a question related to full polynomial function of two variables. I can write code to calculate the value of it but I dont know how to plot the contour at level 0. For example: given a full 5th-order polynomial

f(x,y) = theta1 + theta2*x + theta3*y + theta4*x*y + theta5*x^2 + theta6*y^2 + ... + theta24*x^5 + theta25*y^5

I already wrote the code to compute f(x,y) given theta, x and y but how can I plot this function

theta1 + theta2*x + theta3*y + theta4*x*y + theta5*x^2 + theta6*y^2 + ... + theta24*x^5 + theta25*y^5 = 0 ?

thank you!