To control weeds and moss on lawns you need to concentrate on the cultural elements of maintaining a lawn as well as possible chemical treatments.
In creating a lawn area you are trying to create the elements and conditions that will suit the grass and not the weeds.This involves a number of key principles.
1.The soil should ideally be slightly acidic to discourage most weeds from growing in the lawn area.Ideal soil pH for a lawn should be around 5.5 to 6.0.
2.The soil pH should not be allowed become to acidic i.e below pH 5.5 because this suits moss plants rather than grass.After a period of time feeding a lawn this will lower the soil pH below pH 5.5 and so you will need to address this with a dressing of lime to raise the soil pH.
3.Mow you lawn at least once a week and box of the clippings and remove to a compost heap as this will help stop any weeds establishing themselves and spreading.
4.Lawn scarification in the autumn and spring by hand with a wire rake or machine will help remove and stop any weeds from establishing themselves.
5.Chemical weed killers are best applied in spring from April to June. Make sure before application the grass and the weeds in the grass are growing as fast as possible.This will mean a lawn feed and watering the lawn if dry a week before application of the weed killer.