There's no 'light' way to do it. For strength training to be effective, you need to be challenging your muscles at close to their maximum capacity (ie. lifting heavy).
As a good guide to how heavy you should be lifting, you should aim to fatigue your muscles in 12 reps or less - the fewer the better. 'Fatigue' means you feel you cannot do another rep with the correct form. Form is important, and while it is OK to start out with lighter weights and more reps while you master the correct form, over time, once you get to being able to do 12 reps, it is time to move up to a heavier weight/more challenging exercise.
You don't need to do a whole heap of different arm exercises. There are basically two sets of upper body muscles - one to push, and one to pull. Pushups are a great upper body exercise (modified, wall or incline pushups are easier substitutes - see the Spark article "You can do perfect pushups" www.sparkpeople.com/resource/fitness_artic
Add in pull-ups/lat pull downs/bent over dumbbell rows, and you have worked most of the upper body muscles.
Also, to add muscle, you need to be eating at a small calorie surplus (say 250 calories per day more than you would need to maintain).