I think it's a good idea to get as healthy as possible, and I also think there are strong indications that cancer can be affected by our diet and lifestyle. I've done a lot of study on this and recently met with the dietician that works with the cancer treatment center I'm using. Basic ideas I've gotten include:
1. There are some foods that are "known" to contribute to cancer, such as too much red meat and meat fats.
2. There are some foods that seem to help fight cancer, such as cruciferous vegetables.
3. Exercise, in the right amount/intensity seems to help.
4. Keeping a positive mental/emotional balance also is helpful.
Since I got the word that my cancer has metastasized about 2 months ago, I've personally been working on all of these. There is a lot of data supporting limiting intake of refined sugar and processed wheat, so I've mostly cut these out of my diet. I'm down to only one or two servings of red meat a week, and it used to be my main food. I now run 6 miles on an elliptical at least 5 times a week as well as spend 3 days with weight training, and I try very hard to push back on the negative emotions that come with a cancer diagnosis. I just got my latest monthly PSA test and should hear back today or Monday to see if it all has helped. At the very least, I feel much better, have great energy, and my mood is much, much lighter than before I started doing this.
RobertC suggested the book "Anticancer", and I second this. I have a copy of the book online and it's been my main source of information about what to eat and do. There is a lot of interesting information and background throughout the book, but you can get a quick understanding of the basics from the summaries at the end of the book. I'm sure there are other sources, and you may want to see a dietician.
Best wishes!
Greg.