Try to get enough electrolytes before training and ignore your personal trainer's suggestions to push hard from the beginning - initially go at 50% of what you think you can do and slowly increase this each new workout (personal trainers often want to show off and just kill the beginners). That way you'd avoid the "muscle week off" after a work out. Also, try to restrict soda, simple carbs and start slowly moving towards protein-rich foods, complemented with vegetables and fruits, so that you can build muscles. I started this a year ago, now my fitness is better than ever.