How to eat healthy on a very low budget?

My family is what most would call "poor" we don't have money to buy groceries most of the time. We eat what we can afford . Fresh vegetables are just too expensive to even think about buying. What are some good, cheap meals that are healthy? I would really like to go on a diet, but I know it will be ruined because we don't have the food that I would need to eat. I eat a lot of chicken and brown rice.
I had a baby in March of 08 and haven't lost the extra weight that I packed on. I feel so "blah" most of the time and I think that having a more healthy diet and excercising daily will be great for me. I just need some help learning what I should be doing, and what I can afford to do.