Underarm odor is probably one of the most embarrassing conditions a "normal" person can suffer from. Obviously, if you have poor hygiene, then that is probably the cause of the smells, but there are a lot of people who have a good body hygiene, who wash and shower just as often and thoroughly as others, who properly use deodorants or antiperspirants yet have more aggressive underarm odor.
There are several possible causes for underarm odor. When you suffer from excessive underarm sweat, also known as axillary hyperhidrosis, then it's very likely that this alone can cause the bad smells. The most effective axillary hyperhidrosis treatment I ever found is on the homepage of this site, and I highly recommend you try the method out.
But sometimes people don't sweat excessively in their armpits, but have noticable underarm odor. In those cases, you might also want to look at your diet. Some foods are known to cause more pungent smells. For example spicy foods like Indian curries and spices, garlic, onions, etc. These are all healthy foods in general, but unfortunately they can cause our bodies to smell quiet unpleasant.
Drinking lots of water throughout the day can also help to reduce underarm odor. Because the water will help your body to flush out toxins and get rid of waste products more effectively. This makes it easier for your skin, because it also is involved in the process of removing waste products. Generally, the "cleaner" you are on the inside, the less pungent your sweat smells. Many people who drink lots of water make a mistake though: they drink the water all in one setting, for example in the morning. What you really want to do though is to constantly supply your body with fresh water. It's best to drink a cup of water each hour. Because if you "flood" it in the morning, but then don't drink water again in the following hours, this will do you no good.
It has been shown that emotions also influence our body odor. If you experience a lot of stress and anxiety throughtout the day, your sweat will smell very different than when your day is full of joy, comfort and relaxation. Keep in mind that stress is one of the causes of a whole range of diseases, and if you find that you have too much of it in your life, you will benefit in many ways from reducing it. Of course, this is more of a long-term approach, and not a quick fix to get rid of smelly armpits. But regularly doing breathing exercises, a little meditation, stretching or yoga will probably have a positive effect on your body odors too.
One thing that has worked particularly well for me is using mangosteen powder. I tried it on a trip to Asia and found that it worked so well, that I bought 100 ounces of it and brought it back home. When I now take a shower, I simply take a little bit of mangosteen powder, put it under my armpits and then wash it off with running water. Afterwards, any smells are gone. (Even aggressive soaps sometimes leave a very subtle "residue smell" on my armpits, but the mangosteen powder simply takes the smell away, is completely natural, and not aggressive at all).
So, I hope some of the suggestions I shared with you will help you. But keep in mind that if you have axillary hyperhidrosis - if you sweat a lot under your armpits - then the best way to do something against your body odor is to fix stop the excessive sweating.