If you're completely sedentary you're going to find even the slightest bit of physcial exercise tiring, so the key is not to run before you can walk.
Start by doing some very gentle exercise - you'll soon see the difference: not only do you burn more energy, you also also bust your fat reserves and you even burn more fat when you're sedentary (yes, that means you burn fat more efficiently when you're sat on the sofa - exercise speeds up your base metabolism and increases the amount of energy you burn when you're at rest).
In calorie terms, a sedentary woman only needs 1800 calories per day max, whereas an active woman needs around 2200 on average, and even up to 2400 calories a day.
Don't sign up for a marathon just yet if you're not used to moving your bum off the sofa, though: you need to take it slowly and get your body used to gentle exercise. This week your challenge is simply to get some gentle exercise doing everyday things such DIY, housework, gardening etc.
To help motivate you, here's what you could be burning just doing everyday tasks:
30 minutes' ironing = 67 kcal
15 minutes' sweeping a floor = 75 kcal
15 minutes' general housework = 95 kcal
30 minutes' shopping at the supermarket = 107 kcal
30 minutes' gardening = 142 kcal