The best time to apply a preemergent herbicide is when you see the Forsythias blooming in your area. That's your clue that soil temperatures are just right. But remember preemergents only work to suppress annual weeds that will sprout up from seed. You'll have to use a postemergent herbicide to tackle the existing weeds. If you're going to use a preemergent herbicide, do that first this spring. It sounds like your lawn is thin, so a preemergent that includes a fertilizer might be a good idea. Then you can overseed with a starter fertilizer after six or eight weeks. Then wait until the new grass seed is up and has been mowed twice before you use a postemergent herbicide. These kinds of broadleaf weed killers work best if they're applied as a liquid. It coats and sticks better than a granular.