Weight Loss Help (High School)

Sorry about the long post ahead of time. 

Okay, so, I debated about coming here to bother you all with my problems but I really don't know where to start. 

I'm 16 and I am way over weight. (270 lb) 

I'm well aware that diet and exercise is the way to go but my problem is that I don't know where to start with these things. 

I have recently (3 weeks) stopped drinking soda and I am thinking about within the next week stopping sweets such as candies and cakes, things to that nature. Basically junk food. 

I try to talk to my mother about it but she can be forgetful, even with constant reminders, so it's hard to stay with anything. 

My main goal is to get back below the 200 line. 

When I started gaining the weight I was getting ready to tryout for Volleyball back maybe five years ago and I believe it was muscle I was gaining. Either way all I saw was the number go up and with that I became more discouraged because my goal at the time was to lose weight through the sport. 

Now I am in band but because it is not marching season I'm not active as I would like to be and I am bombarded with homework. This makes it hard for me to find the time to throw in exercise. 

Especially because I'm self conscious on trying to exercise when people I know are around. It actually makes me embarrassed that I feel this way about trying to better myself around the people I know. 

Sorry about the life story.