Lose 10 pounds and keep it off.

ok
so I slept until four yesterday, and saw I was 145.5 lbs!
but then, after I had dinner, I was 146.5, still not bad.
Then I had lunch today. When I got back, i was 150!
I don’t understand!
how can I gain 4.5 lbs in less than a day!
I feel like crying right now.

I just signed up for this today.
Here goes nothing.
I’m 14 years old, 5’8, and 149 lbs.
I know that my friends would never tell me, because they’re afraid of hurting me, but I need to lose 10 lbs.
I am going to lose 10 lbs by new years.
That means 1 pound a week.
which means cutting 500 calories a day, since 3500 cal=1 pound.
I plan to burn an extra 250 and eat 250 less.
This way, I know I’m burning fat.
My biggest problem is weekends, because I have nothing to do most of the time.
Sure, I go out with friends and shop, but I spend about 10 hours sitting at my computer or watching TV.
So I’m going to take up tennis with my friend Katrina.
I’m going to have a 1 hour lesson every week and practice 2 hours a week (1 hour on saturday and 1 hour on tuesday)
That’s 1100 calories a week, plus my lesson, which should burn around 300.
I guess I’ll see where I go from there.
I’ve tried to lose weight before, but I always mess up or give up.
I know that I can do it this time.