How To Get Rid Of The Bad Odor From The Underarm?

Hi, i am stef 12 years old. i have a problem with my underarm and it makes me feel uncomfortable when i'm sweaty. My underarm has bad odor, and i just wanna ask how to get rid of it. Actually, the bad odor in my armpit just started when our old maid joins her clothes to our clothes when washing them. And i noticed that she have a very bad odor in her armpit. I tried crushed alum, but it don't work. can you please help me in my problem?