Advice on becoming a boxer?

Hi i've always liked watching boxing but now i'ved decided that I want to learn the art and become a boxer. I'm 19( a little old?) and need some advice on how to start since i'm on my own with this right now. I have a punching bag and i've been doing 8 3 minute rounds for now. Other help I need is on weight loss. I work out a lot 5 days a week weightlifting and cardio but how can I lose weight without losing much muscle? I can lose 20 pounds in 2 weeks easy but I don't want to let all my weightlifting hardwork go to waste. I'm 5"10 190 and my real goal is to get down to around 155 for boxing. Any good advice would be appreciated.