This is definitely a unpleasant situation but can be managed naturally.
It is the bacterial growth
in sweat which results in the odor.Do the following
Shower with warm water daily which will kill bacteria in armpits
In summers take shower twice daily.
Use a anti bacterial soap to clean armpit
Wear cotton clothes
Avoid eating garlic and red meat
Regularly shave your arm pit area
Use any deodorant in day time
Use anti perspirant containing aluminium chloride n the night time and wash in the morning.
You shall be all right soon.