Yes you need to kill any weeds before adding top soil otherwise they'll just grow through. If you use enough top soil, you can probably get away with not rotavating but if it's only a thin layer in places, it would be best to rotavate as well.

Echoing the above!

As a professional horticulturist I would recommend that you weedkill and then plough or rip the ground to get rid of compaction. If the existing soil is compacted (by wheels, feet etc) water will not percolate through and when you put the new topsoil on, the topsoil will be sodden in wet weather and extremely dry in warm weather. Once the compaction has been relieved, carefully spread the new topsoil and reseed. If you don't look after the fundamentals - the soil - you will never have a successful garden.

Don't use weedkiller in any circumstances, it pollutes your environment, kills insects , bees, mycelium etc and gets into the food chain. The best way to get rid of weeds is to alter your attitude to them . The definition of a weed is an unwanted plant. So ... here,s what to do.Dig out any large perrenial weeds. As previous comments , rotovate soil to remove any compaction. Add topsoil and add sharp sand. So here,s the thing , if you regularly mow your lawn i.e. once a week , it will kill the weeds, because unlike grass most plants cannot sustain being repeatedly cut back,and don,t worry about the odd weed here and there