Welcome to GOY Enda it depends what you used to kill it with and when you intend to plant in it? Some weedkillers may last 6 - 18 months or more check the instructions.
If it were me I would lift and store the dead turf in a pile to rot down slowly and dig the remaining soil over roughly and let the frost do the hard work and break it down for you. I would also add lots of compost and other goodness to the soil ready for spring planting of your chosen perennials?
It might be a good idea to do a soil test and look at the overall conditions of your garden so the plants you choose suit them sun, shade, wind, claysoil etc.. Also look at what your neighbours grow it will give you some idea of what grows well in your area and help you decide if you like the affect or not.
I hope you find this fun as new beds are always a joy to do.

Thanks for that your help is apreciated.