When I first noticed a problem, I thought it was just my armpits. I have hyperhidrosis so that made it a thousand times worse. My freshman year of highschool, I spent most of the day with my arms shut tight by my side. This obviously didn't do much but that's the only thing I could do. Everyday was hell because people can clearly see what I was doing and could likely smell me if they were in the same room. Towards the beginning of Sophomore year, I bought a product called Odaban. This stopped ALL sweating there and helped out a lot. If I was sweating, I'd sweat from my chest and back but my armpits would be dry. The weird thing was, I'd still have a pretty bad odor. I went through the whole year just trying not to sweat / exert myself but living in a desert this is pretty tough.
About 2 months ago (still a sophomore), I got the idea of putting deodorant on my chest and back as well as my armpits as part of my daily routine. I'm pretty sure this is what changed me from "I can stay fresh for like 2-3 hours" to "I can stay smelling GOOD ALL DAY." At this time was when the compliments on my cologne started rolling in. This was the time I could be comfortable in my skin and lift my arms over my head without thinking twice. This was the time I could walk around the classroom without dreading every second of it. I could be hot as hell, sweaty thinking I'm going to be musty and I get complimented on my smell.
Another absolutely KEY thing is to get yourself a gym membership. They have showers there so if you're REALLY insecure, you can shower mid-workout. Get yourself sweating. Run a few miles, lift some weights, relax in the sauna, run a few basketball games, take a few laps in the pool. Whatever you do, just get yourself going. This is kind of two birds with one stone, you are making your body look better and helping treat your BO.
EAT RIGHT. I can't stress this enough either. If you put crap into your body, you will smell like crap. Cut the burgers, sodas, pizzas, candies. Put in real fruits and veggies. Google foods that make you smell bad and the foods that improve your scent.