It sounds like you are giving yourself too many things to change at once. Try doing things in smaller steps and maybe make a chart! So, from Monday, you are going to make sure you tidy your room. No excuses, Monday night is room tidying night.
Prepare for it- put on some fabulous loud music, get some nice smelly spray and some cleaning stuff, and go crazy. Tidy and clean for at least 2 albums before you are allowed to leave the room for any reason! Then when you have achieved that small goal, give yourself a reward. Time it so you can watch a film afterwards, or Eastenders, or whatever, something to look forward to.
Once the room is really clean, try your hardest to pick up after yourself to keep it nice, but then do another blitz in a months time.
Do the same with the homework. Set yourself a WRITTEN homework timetable. A set subject and amount of time per night, and set aside a time to do it.Tell everyone at home about this so they can encourage you and make you go and do it!!
Healthy eating is hard without help. Your family (if you eat with them) need to do this too. But make sure you have a good supply of NICE fruit and take an apple and banana to school with you to have at break time. That in itself will really help change your palatte. Drink lots of water. Get your family to eat more veg at dinner time.