every day i have a different weight !!

Normal weight fluctuations. They even have weight trend apps so you can watch your weight over time rather from day to day.

Trendweight
Libra for Android

I'm sure there's others.

Fluids, food, glycogen, hydration, time of day (morning vs. evening) evacuated vs. non-evacuated 'stuff' - all play factors in what numbers show up on the scale. That number looks like 'weight' but it isn't really 'fat' which doesn't come and go that easily. If weighing every day changes how you're feeling about weight loss and your commitment levels throughout the week then I would recommend staying off the scale. Weigh in once a week and look at a 4-week trend at a time. Even the same weigh-in day weekly isn't a guarantee of consistent downwards (or even upward) trends. You definitely need to focus on consistency, look at the weekly weigh-in as feedback only and a snapshot in time and see how things look over a few weeks.

Do you weigh yourself at the same time each day?

Is your scale on carpet?

My recommendation is to either weigh yourself daily and use one of the trend apps mentioned (I love Happy Scale) or weigh yourself monthly. In the beginning, daily with an app is probably better since you're still figuring out what works for you and don't want to go a whole month without feedback. Weighing weekly is the worst of all worlds in my opinion because the data will be misleading depending on your water weight on that given day or if you have undigested food in your stomach. It's depressing to wait a whole week to step on the scale only to find you "gained" a pound (but in reality if you'd been weighing yourself daily and calculating the trendline, you'd see you actually lost 1.1 lbs).

Also this is probably obvious but always weigh yourself either naked or wearing the exact same clothes and at the same time of day before eating or drinking anything. It can make up to a 5 lb difference when I don't!

You can graph your weight on MFP and just kinda mentally note the trendline.

Yeah, body weight isn't static...nobody weighs exactly XXX Lbs. The human body is made up of anywhere between 50-65% water and it's variable and obviously, water has mass and thus those fluctuations would show up on the scale. You also have variable levels of waste in your system at anyone time, etc. Weight management is about overall trends, not the weigh in to weigh in numbers.