If the ground is covered with weeds, it may be easier to treat with a glyphosate weed killer and leave for a few weeks, but whether you are sowing seed, or laying turf, the ground needs to be prepared properly. For a well drained lawn, you will need at least 6” of top soil. Less than this and the area will become swampy. If necessary the sub-soil will need to be broken up and sharp sand incorporated. Dig the area over, removing perennial weeds such as dandelions and couch grass. A bit of hard work at this stage will pay dividends later. Rake over the area to get it as level as you can, removing large stones, then firm in with your heels, or the back of a spade. Do not use a roller as this will compact the soil, encourage moss and cause oxygen deprivation. You are not trying to create a cricket pitch. Rake again very lightly before you sow the seed. Do not incorporate any kind of fertiliser at this stage, it is totally unnecessary.