Weed preventer is best applied in Spring just before most weed seeds germinate. When you plant a new lawn in spring, you cannot use weed preventer because it would also stop your grass from germinating. That's one reason why fall is a better time to start a lawn from seed -- most weeds don't germinate and grow at that time of year, so you can get your lawn going without having to fight so many weeds.
OF course, that doesn't do you much good now! But next spring, be sure you use a preventer (Like Scotts Plus Halts or equivalent). This fall, you can clean up many of the weeds that grew this year with a fertilizer + broadleaf weed killer. (or you can spray a broadleaf weed killer, which is more work, but also somewhat more effective).
That should do it pretty well -- fertilizer + broadleaf weed killer this fall (and an additional late application, like in November, of winterizer will also help a lot), then fertilizer plus weed preventer in spring (early/mid April in your area).