Everyone has pretty much out thought what I could suggest you to do Polaris, which is why you need to start taking their advice seriously, my worst fear of life is hearing about a friend passing away because they did not take care of their health. After reading the first few paragraphs of your statement I actually scares me especially since being over weight is one of the most reasons for... well passing away, usually because the heart just can't manage.
Like stated, you need to become more motivated to actually do something about it, it's not something we can do for you but we can provide suggestions that you can proceed with. Some are more drastic than others... and some solutions may or may not work for you. But you have to start some wheres and where I would start is stop eating sh*tty over processed foods!
Actually snacking in small amounts is better for you than eating full large meals, food such as nuts, fruit, vegetables, yogert and pwheat products are what you want... but it has to be NON PROCESSED stuff such as that you find at most grocery stores... you just have to look at the ingredients. Stay away from pop especially and particularly DIET POP, no sugar doesn't mean it will be good for you in any way, it is actually worse I have heard. Learn to read the ingredients of food products, if the list is long then just stay away from it. Look for low sugar, low transfat and low sodium food products.
This is a start... and it might scare you at first but trust me... you will not only start losing weight but you won't feel like a greasy ball of nasty... the sort of feeling I get after eating at fast food places like Mcdonalds... stay away from grease, if you eat out look for places that actually sell real food not deep fried sticks of zombie flesh.... sorry Zombie. As for fruit juice, look for non concentrate, for example; Terra Beata cranberry(if you like cranberry at all), it literally has 3 ingredients in it, water cranberry juice and sugar... no preservatives what so ever! I then mix it with carbonated water and orange juice and to me it is like pop.
Subway is ok... probably the healthiest of the bunch even though everything you get is still over processed... might as well be an earth pony and grow your own food, trust me it will taste better. That being said my parents pretty much buy everything local and as a result it is fresh, preservative free and tastes even better than what you can get at most grocery stores!
I do like walking, so if I take walks and try not to eat enough to kill a horse, will that be enough to unfat myself?
Now for excersise... walking is the healthiest and least stressful form of exercise, it circulates you blood and helps with your breathing but also puts minimal stress on your leg and knee joints unlike running... I have knee problems myself, and bad circulation in my hands and feet but I walk every wheres and for me a 3 mile walk is nothing. I don't even have a car or drive!
My story on how I have stayed healthy is basically because my parents grew me up this way so when I do eat fast food or junk food of any sort I can't eat a lot of it because I get heart burn and such.... I would discuss this with your parents.
I myself am 5'7" and weigh about 145 pounds... I'm practically a twig and as such I'm slightly under weight according to my doctor... I thank a high metabolism and the kind of career work I do for that but I think for you your actual healthy weight would be around 200-250 that is only a guess but make 350 your goal first as crazy as that sounds.
Phew... I hope this will help you make a good choice, if this long wall of text doesn't scare you away.