To prevent weeds from harming your lawn you will have to start from the beginning, since the time you start growing a lawn put down some weed killer material, than wait the necessary amount of time prior to planting the seeds.

A weed controlling fabric is very useful for small sections of lawns. Often lightweight portions of cloth (usually black) which are intended to minimize weeds.

If you already have a establish garden or lawn if is possible to control any weeds successfully by using a pre emergent herbicide. Which basically means that the chemical used will destroy the weeds at the seed stage, rather than waiting for them to sprout.