As a Personal Trainer in training i can advise you on simple methods to help up your metabolism without going to the gym.
As another person mentioned drink more water, 1 litre per 25 kg body per day, plus another 1 litre per hour of exercise you do.
Try to involve more incidental exercise, when you go to the fridge for water, turn around go back to where you were then go back to the fridge. Put a low stool / dinning chair in front of your TV and stand up / sit down while the ads are on. Park in the middle of the carpark instead of right by the doors at the shopping center ( i dont recommend the edges as i dont know your neighbourhood and it may not be safe ). Really think about things you are doing in your day to day life and try to allow an extra 5 minutes to everything, so you can take the long route when walking somewhere / take the stairs instead of the elevator.
If you crave food, try not to have any for 30minutes, try to find something else to focus on. If you still crave it then have 1 BITE not the whole packet...
Try to eat more fresh vegies, not frozen ones, preferably something you have grown yourself and it is straight of the vine. A still growing vegetable is so much more healthy for you then any frozen / processed one.
Eat more meat, preferably fish or chicken ( free range ). A high protein diet is proven to help speed up your metabolism.
When eating foods remember: Alcohol is burnt first, then Glucose from sugar, then Carbs, then ( stored ) fats, Then proteins. Your body should be using its proteins to repair muscles and such, even in the day to day grind ( wihtout specific exercise ). Proteins are never stored as fat.
So you CAN help yourself even if you dont exercise specifically, you just need to be aware of choosing to help yourself.