Are you 'very' or 'slightly' overweight? The reason I ask is that if it is the former I would set yourself a weight target to reach before hitting the gym. This provides an additional challenge and helps to keep you focused.
I have been on many successful diets but haven't once been to gym. I mention this as eating healthily is always the best option.
I'm not sure you can match what foods to eat with having exercise. If you were trying to build muscle mass then I suppose protein rich food would suit. But if you are wanting to lose weight then the relationship isn't so important. Take in less energy than you use up and you will lose weight. Eat a balanced diet.
Running a marathon uses the energy equivalent of 1 large loaf (800g) this is approximately equivalent to 300g of fat. Gym training does not expend energy at this rate so 1 hour in the gym cannot lose more than 100g (3oz) of fat. It is a lot easier to eat less and it actually costs less.
Muscle is heavier than fat. If you don't have much weight to lose I would suggest measuring yourself as well as weighing yourself. You might not lose pounds on the scales but you can easily lose inches instead.
Before going to the gym I used to eat a ham salad roll and a banana.
Well, let us take someone who goes to the gym to lose weight.
They have a ham salad roll and a banana before workout..total weight lost..NIL.
Defeats the object.