How much weight do you want to lose before school starts? Safe weight loss is 1-2 pounds/week, so the most
you should plan on is 10-12 pounds, if you stick with a pretty strict diet and exercise routine, but that's a good start and if you stay with it, after school starts, you can really make progress over time.
There are numerous exercises for belly fat, such as crunches, leg lifts, even brisk walking. Try searching YouTube for "how to" videos.