Start by increasing water intake A LOT and also increasing fiber. It takes a while to detox from junk food, but eat as many veggies and fruit as you can. If it's not a daily habit, start by using a fiber supplement. Staying properly hydrated and fiber intake will shed the first few pounds very fast and you will start feeling better!!
Drink more water, don't ear carbs after 6.30PM, eat more fruit and veggies instead of sweets (I know this sounds generic but it will work) and eat 5-6 small meals rather than 3-4 large ones.
Also another alternative is a vegan diet. I'm not a vegan but occasionally eat only vegan food for a short period of time. There is very little fat and NO cholesterol!
My advice is:
Drink more water - 8-10 8oz glasses spaced through the day
Make your protions smaller - often its easier to use a smaller plate - it fools the mind.
Increase your protein intake and lower your carbs (don't cut them out)
Don't eat or drink for the 3 hours before you go to bed.
Make sure that you do some form of cardio exercise for 30 minutes straight every day - brisk walking is fine.
Make sure that you MOVE every hour - even if it is around your home, up and down stairs. Just dont stay still for more than 50 minutes uness you are in bed asleep.
Keep snacks to a minimum - allowing the body to rest between digesting meals helps.
When you feel peckish or hungry drink water before reaching for food.
Remember it really is about calories in and calories out - so by cutting down the calories you put into your body and moving about more you can make huge changes.
260lb is a lot to lose and looking to do it in 17 months is a very big challenge - I think that you should consider professional help and should have your overall health looked at by a doctor before starting any radical changes to your lifestyle. You can do this - you cna make the changes you need but you will need a good support network around you.