Mom - getting rid of the pop will help, but each day cut just cut one from your diet. Let's say you drink 10 a day. So tomorrow drink 9, then 8 the next, then 7 until you cut back to 1, then do a half, then a quarter and then bingo you're off of the caffeine will very little side effect to it. So what if it takes two weeks, it's better than suffering and you are improving each day.
As to the gym. I joined one in April. There are people of all sizes there. I end up going middle of the day mostly as that is what works for me, so maybe a different hours you would see different things, but on average, I see about and equal amount of heavy vs. thin. Also, if you join a gym that has personal trainers, they can give you a good program to get started with, its really not as bad as you think.
Another poster mentioned who gets the flack at gyms and I believe they are very right about that. I've never seen or heard a heavy person getting any flack at all.
you can do this, don't be afraid, it's a lot of work, but very doable.