The most effective exercise is the 1 you enjoy doing most.

To build muscle, you need to lift some weights.
To lose fat, you need to meet calorie deficit, decrease your calories in, increase your calories out aka your overall physical activity level.

For us to give you a better suggestion,
you need to tell us more about yourself, your lifestyle, how much time can you commit for physical activity, what you eat on a general basis.

But best, is to consult a professional to give you a tailored advise.
Rather than listening to people here tell you to walk 3 hours a day, eat once a day.