To be honest, I think 100 is a little low. I'd aim for 105. It will probably be a lot easier to sustain.
If you excersice and eat smart, you should have no problem achieving this by maybe late June, early July - since your young and your metabolism is fresh.
First piece of advice - don't start counting calories! It's a crazy obsession that will drive you mad, believe me. The best way to start is to "eat with your head, not with your stomach". For example, if you replace ONE unhealthy thing you eat in a day, eg. McDonalds, Pop Tarts, etc etc. with one healthy thing eg. an orange, an apple, etc. you'll already see a big difference. DO NOT leave out any food group - all this junk about cutting out bread/carbs is RUBBISH. --Everything in moderation-- is the key. If you really want a number though, I'd say between 1600-1700 to lose weight stabally without excercise, but more if you excersice.
The biggest mistake people make when trying to lose weight is eating too little. Not only will you gain whatever you lose back, but you wont lose as much! NEVER EVER EVER go under 1500 calories since youre under 21. It'll screw your metabolism SO BAD. Ok :)
As for excersice, try going for a jog 3 times a week for 1/2 an hour or a walk for an hour 3 times a week. Nothing excessive. If you're looking to lose weight in general is cardio (eg. walking, jogging). And its not tough either, can even be fun :)
HEALTHY, sustainable (eg. you wont gain it back) weightloss is 1-2 pounds (lbs) a week. Sometimes a little less. If youre gaining more per week, EAT MORE. It's not healthy or good. Those people who say they lost 15 lbs + a week are doing unhealthy things.
One final hint that I found helped me - if you drink pop or soda, replace that ONE soda a day with a bottle of water. Its a small thing that helps a lot, esspecially during weightloss.
Good luck, hun! And remember, stay healthy!! :)
I know wayy too much about this stuff ;)