Weed and seed or weed and feed? I'll assume the latter, since weedkillers and seed don't go together, so any of the proprietary brands will do, but you may need to spot treat the dandelions, it won't get rid of them completely. If you have moss, you may need a weed, feed and mosskiller formulation. If it needs cutting, cut it first, but check the pack of whatever you buy - some say to apply when the grass hasn't been cut for a few days, others don't. If its a granular formulation, it should be applied when the soil is moist, the blades of grass dry, and if it doesn't rain for 4 days (again, depends on the product) you have to water it in, so best to choose a time when its dry, but with rain expected.

Yes agree with Bamboo, those tough perennial weeds like dandelions and thistles etc can be spot weeded I use Verdone spot weeder it's very good and normally kills the roots completely of dandelions etc.