Fresh vegetables are great, and frozen are fine. All offer a lot
of nutrition, and raw and cooked vegetables should be the mainstay
of your diet. I think it's fine to use dressing, but just consider the amount of dressing you use if you want to lose weight.
The first week you may not feel good, but then you will start picking up. New habits are hard to make, but worthwhile. Start a walking program and get out in the sunshine. You can feel better.