See whether your week adds up to a calorie deficit or surplus, not just today.
Days don't all go to plan. A meal out on Saturday doesn't ruin a week if Sunday and Monday are lighter, and your weight responds to the overall balance, not any single day. My Calorie Pal adds up the calories you eat and burn each day and shows the balance for the whole week, along with the weight change it suggests.
Calorie deficit 101 is a short, plain guide to how weight loss works and how to track it.