The answers to these questions depend on a lot of factors. What type of lawn do you have? Certain weed killers are labeled only for certain types of grasses. Are the weeds grassy weeds or broadleaf weeds? Look for a weed killer that kills the type of weeds that you have.
If you're wanting to completely kill some of the grass to plant flowers, use glyphosate (Roundup) to kill everything in the area that will become the new planting bed. Wait a week to 10 days, then dig or till up the planting bed, incorporating organic matter such as compost into the bed before planting your flowers.