| It honestly sounds like you are on the slim side man. I would avoid cardio-only exercises entirely and focus on strength training and weightlifting. Done properly, these can give you good cardio exercise while you strengthen and grow your muscles.
Try 3-4 days a week and start out slow! You may find yourself gaining weight, assuming you get the right nutrition, but your body will appear denser and more compact than perhaps it even does now at what would be a lower total body weight.
This is how I would start, never working out more than 2 consecutive days and not hitting one muscle group more than about once a week (to start):
Day 1: Chest, Shoulders, and Tricep exercises
Day 2: Bicep and Upper Back exercises
Day 3: Legs and Lower Back exercises
Day 4: Some varied sort of exercise. Play basketball or raquetball. Go hiking. Jump rope. Punch a bag for while.
If you increase your protein intake sufficiently, get 7-9 hours of sleep each night, and consume enough overall calories, you will gain significant amounts of muscle. This will also give you an increased metabolism, much lower insulin requirements, much higher insulin sensitivity, increased levels of testosterone and growth hormones, a stronger immune system, and a host of other benefits.
If you want to add cardio in, go for it, but you will need surplus calories to build muscle.
Increased muscle mass is exactly what eliminates flab and the flabby look.
Good luck and let me know if you have more questions!
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