Be sure to shower or bathe daily. Use proper body cleansing agents such as soap or shower gel. Wash everything completely especially your underarms.
Don't use antiperspirant/deodorant (except in cases of excessive sweating). It will actually end up making you smell worse than ever.
Keep your underarms dry. Bacteria have a hard time breeding in dry areas of the body.
Shaving your underarm regularly will help prevent the accumulation of bacteria and can reduce sweat and odor.