You need to take shower one-two times a day ( maybe and more if your lifestyle is dynamic and You sweat a lot). Also use a deodorant every time you can. If this doesn't help then consult with a doctor.
Start by making sure the odor is not coming from your clothes. Mix baking soda and water into a paste and spread under the arm pits of your shirt. Let dry 24 hours and the laundry. For your body look at what you are eating.