By losing weight i assume you are trying to be healthy, because a low weight ≠ healthy. your name says you're a girl, so i suppose you are going for the lean, tone look.
The most effective by far, for losing fats and becoming tone - lifting weights. Lifting weights does not mean curling dumbbells like the bodybuilders, but going for compound movements that work your main core muscles, that include bench, squats and deadlifts. The common myth about lifting weights is that if a female do it, she will become big and gross. I assure you woman has a low testoreone lvl and gaining muscle mass is one of the hardest thing for a female to do. Those big woman you see often are most likely on juice (steriods)
Another alternative is crossfit, which often combines cardio and weights training into a functional training package. You might need to do a little more research on your own.
Yoga is the most suitable for those already thin ladies who claim they have flabby arms. the main exercise you get from yoga is the sweating - extremely effective in losing weight! but is all just water.