First of all, if you have arrived here from scanning a QR code on a card you found, it's because there are people who care about you and want to help you improve yourself. These cards are often given anonymously, as it is hard to tell someone that they smell, but life is better for everyone once you know and can do something about it. Do not worry about who has given you this card, just receive the feedback gracefully and take this opportunity to do something about it.
This is important. It affects your relationships negatively and should be addressed.
Here is a list of resources of various sorts that talk about what body odor is, why it happens, and what to do about it. The list is in no particular order, we recommend that you look at each one and find the ones that you resonate with.
You can also do your own search for "how to reduce body odor" on any of the popular search engines.
We wish you the best of luck on your journey