Functional abilities will change from the very first session and continue to manifest going forward. Your fitness and vigor will increase immediately and in relation to training frequency and nutrition! Visible changes in your body can take up to a month to show through and are largely diet dependant.
Remember: our results are stimulated by what we do in the gym, they are physically allowed to happen due to the diet you follow: if you eat poorly then your results will be limited. If you eat whole foods full of nutrients appropriate to your goals, in the correct quantities then your results will be immeasurably superior to those with poor non supportive diets.
It can't be overstated just how important, no, essential, a supporting diet is to your results. The diet you follow is simply another way of saying the food you eat, some diets are weight loss, some are weight gain. All can be very simple at their core so don't be daunted by food.