Working out and cardio will only get you a tiny amount of fat/weight loss (15-25%). The best way would be via the diet (if i had a nickel everytime i say this, i'd have a s**t load of nickels) to lose fat/weight. Write it down and see the totals of what you consume (fats, carbs, protein, etc) There are many diets out there but i feel the best way, at least for me, is carb cycling.
http://magazine.mindandmuscle.net/page.php?pageID=159&issueID=15 by Twin Peak
Why only arms? If you are serious, you should do more than that. Try starting with a 3 day beginner split.
Workout 1: Chest
Workout 2: Back
Workout 3: Legs (yes legs, this will help you build your chest & back from the higher hormone output)
After a while you can split it up even more, like a 5 day split
Workout 1: Chest
Workout 2: Legs (upper legs)
Workout 3: Bi's & Shoulders
Workout 4: Tri's & calves
Workout 5: Back
Or whatever split works best. The difficulty is making a plan for you and that you dont work a body group 2 days in a row. IE, doing chest one day and then the next doing Triceps. You dont want to do this since Triceps are used for chest.
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