I'm new here so sorry if I'm a bit rambling or this post is in the wrong place

I'm 16 years old, , 5 foot 11 inches tall and 210 pounds in weight (15 stone or 95.25 kg). Using the BMI scale that puts me at the high end of overweight and nearing obesity (I realise that young people are told not to take BMI as seriously but I can tell from appearance and fitness that its verdict is correct).
I have gone up and down in weight over recent years and I worry that this is partly due to difficulty with phases, i.e. getting into one thing for a period of time then losing interest and moving on. The most recent and extreme example of fluctuations is change in weight and fitness over the last year. Last year I went to the gym regularly (2 - 3 times a week for just over an hour a time occasionally followed by swimming) mainly using the bike, cross trainer and circuit training machines and was part of a karate club where I was of a high level and trained twice a week. This saw me drop in weight to a healthy weight (a relative achievement for me) and eventually fall to a point where I had a 'normal' appearance and could fit nicely in waist 30 inch shorts (again an achievement for me). This was largely due to me combining exercise with a strict diet based around the one mum was following for Slimming World. Unfortunately, as the summer holidays of last year approached I began to succumb to cravings and slowly returned to unhealthy eating habits and lower exercise levels. That has now continued to the point where I have gained possibly over 3 stone in weight, lost a good deal of strength and fitness, quit karate club and regularly skip gym, only doing relatively light and ineffective exercise for short periods if I do attend (I also no longer swim at all). I left that karate club due to a combination of overall issues with the club and comments on increased weight from the instructor; I stopped going to gym due to apparent loss of motivation, but continued to not attend properly due to a loss in confidence and a high level of insecurity due to how I look etc. (this was of course not aided by the fact that gym wear no longer fits properly and I haven't wanted to admit the need to buy a new set (this not aided by poor dialogue with mum on the issue of weight)); I have also refrained from swimming due to the mentioned insecurity etc and pants size has risen from 30 to 36 inches.
diet is not awful if you take it without after/before/in school snack additions, with breakfast consisting of either a mix of yogurt, fresh fruit and oats/granola-like stuff at anytime from 6:10 to 7:00, lunch consisting of a sandwich (with brown bread), a pack of three Go Ahead biscuits and an apple (although I often fail to eat the apple) often eaten at break rather than lunch at 11:00 to 11:15 and a dinner of a healthy meal if at home due to mum's diet (varies but often meat with veg, beans and rice or pasta) or a less healthy meal if at grandparents' (anything from toad in the hole to battered fish and chips) at 16:30 at grandparents' or 18:30 to 19:00 at home (often not followed by pudding unless a treat at home or followed by jelly or ice cream at grandparents'). The snacks are the awful part: depending on mood and the availability of change I can buy cheap junk either before or after school or occasionally both, eating most of it myself. Over the last week I have calculated the calorific value of these snacks and it can often come above 1000 and sometimes above 2000 - the only reason I haven't ballooned further is because these snack days are balanced by days when I lack time or money to buy extra food. Another awful facet of this system I've developed is that I often keep family unaware of snacking so that when I get home I can convince them its OK for me to have more snacks.
I apologize for the long and perhaps unnecessary backstory but I feel it useful for me to get all this down on paper so I can appreciate all the flaws in lifestyle.
The specific reason I have picked up the momentum to come on here and solicit help is that I will be leaving school for study-leave sometime next week, with that leading up to exams which will then be followed by a long holiday (10 weeks or more). I had a discussion with a sporty 'friend' at school today who said he'll be joining a gym different to mine which is also cheaper but still close by and I feel this time away from school could be the perfect time to get myself on a better path. I could switch to this new and cheaper gym, hopefully removing some of unfounded fears of people realizing massive shift in appearance (I realise that they really wouldn't care but I still feel anxious whenever I go to attend) and allowing me to occasionally meet up with this guy for some motivation. I would have time to start doing more regular exercise (with every day of the week free accept for the exams) and could possibly even walk to the gym as town is only 45 minutes away (max) for me at walking pace. procrastinating and mainly academically focused nature would normally lead me to leave this until after the exams but I worry that that could lead to me taking the excuse of the end of exams to do very little and then continue to do very little, potentially gaining weight, over the summer.
I would like your help mainly on exercise regime and general tips for keeping going over a longer time as this is where I've failed in the past (perhaps the incentive of eventually appearing more muscular would serve as motivation as previously I've never achieved a very defined appearance). I will willingly accept diet tips but will be largely constricted by mum's diet and grandparents' cooking as I still rely on family for food. I would love to attempt attending gym 5 times a week in this off-time, although it may not always be achievable. Extra detail on execution of exercises or where to find details would be good especially if anything involving weights is involved as other than some dumbbells I keep in room I have no experience with that type of exercise.
Here's a few pictures: http://i.imgur.com/wrL88bu.png http://i.imgur.com/XvZmi2x.png
If I manage to maintain momentum/motivation and start on way with getting in shape I'll use these as the first of a set of pictures to track progress.
Thanks in advance for any wise words and sorry again for the long text.
