Hi, Welcome.
You are probably not going to like this but achieving your goals will take more effort than it sounds like you are willing to put in. But if you really want this then we here can give you as much guidance as possible.
I think the first thing you need to do is read the stickies at the top of the nutrition and weight lifting forums. Nutrition 101 and Weight Training 101. These should help give you an understanding of what you will be getting yourself into.
Believe me when I say this, it is going to be hard. You CAN make a difference without cutting out every single bit of "unhealthy" food, but it totally depends on how far you want to take it. You would be better trying to make changes that you can stick to and make these things life long changes to improve yourself permanently.
I would say cut out the diet soda, every so often isn't so bad, like maybe have 1 or 2 on the weekend. With a crap diet you really won't make much progress regardless of how hard you work.
Read the stickies and do a search for "full body workout" in the weight training forum. You will find plenty of suggestions to get you started. Do you have pictures of how you look now and a goal you are aiming for? Maybe a picture to give an example?
If you really want this enough and make the changes required to your lifestyle then you will reach your goals and even surpass them, but I can not stress enough how much effort it will take. If you start and think it is going to be easy you will give up because it will not work.
Let us know when you have read up the stickies and post any questions you still have

Can you make it to a gym 3 times a week? Also post what your diet usually consists of and then we will try and help you by pointing out what needs to change. If you are new to this it may come as a shock how much effort is needed. I know I have said that over and over but without the determination you will not get anywhere.
I would assume this guy is new, and he is also only 16 so this is probably the first interest he/she has ever taken in their fitness. You could have worded your post better and made is helpful to the OP rather than telling them to stop using the wrong terms...it is right for people to learn the correct terms but you could just say it in a more helpful manner, instead of in a way that may put a new comer off completely. So come on 'dawg' it doesn't take that much to be a little more friendly...surely? We all have to start somewhere..
Cheers,
Dan