Lung cancer - having surgery in 4 weeks. Losing appetite but need to maintain a healthy weight. How can I do that?

I’m not sure what to say really. I am awaiting surgery for the removal of my top right lung and a section of the lower part of right lung. I know I need to keep fit and healthy but how do I eat the right foods and snacks? I have not lost any weight but my appetite is beginning to slow down. What can I do to maintain a healthy weight and physical shape. I have 4 weeks before my surgery. Thank you for any advice.