Contour Plot in R

I am fitting a non linear model to my data. I have two parameters to estimate i.e. b1 and b2. I wanted to plot the estimates of parameter b1 and b2 where each contour on the surface is a line of constant residual sum of squares. I am trying to plot this contour in R. Can somebody help me out to figure how to plot this countour plot of residual sum of squares for different estimates of b1 and b2.

Thanks for any help. I appreciate it..

Binod Maharjan
Lubbock, TX:)