Nothing ruins a lawn like weeds. Sometimes, pulling weeds out by the roots or laying down a stifling layer of mulch is enough to kill them. But if that doesn't work, you may need to resort to chemical warfare--weed killers. In that case, consider the following in your plan of attack:
1. Don't spread a weed killer on a newly seeded lawn--it may do more harm than good.
2. If you spray on a weed killer with a garden-hose attachment, wait for a calm day with little or no wind to prevent it from spreading to adjoining flower patches and harming them.
3. If you mow weekly, spray weed killer in the middle of that cycle so that the killer can sink into the weeds and do its work undisturbed.