Personally, I'd do exactly as Bookbook said, and have a scoot round for anything that you can harvest now or that needs clearing around it.
I'd start now and cut down everything else, and go square meter by square meter over the autumn and winter, dig out all weeds and cover immediately after with cardboard. Don't let any soil sit bare over winter. Weight the card down by [if you have one] mowing all the weeds, or by using heavier stuff [stones, bit of wood, bricks] to stop it blowing about.
I'd not put anything in now, but just clear and plan, for next spring.
I'd decide on whether you are going to have permanent paths, and where these are going, and plan that into the clearing up as you will want to do that first, so that you can put [for example] weed fabric down and stick to walking on that, whilst you clear the rest. The beauty of this approach is that you can grow more in the beds, than you can if you don't have paths and grow the traditional way, where you have to dig the whole allotment over each year. Also are you having a shed? Are you having to collect your own water? this really needs careful planning so that you don't put it in the wrong place.
Also what type of soil is it and does it need anything adding? If you have thick clay soil, you might want to think about how you are going to add organic matter, and are you going to mulch everything next year to stop it drying out? Set up an area where the weeds that you dig up can be composted, I use dalek style compost bins, but as I have clay soil, I put them on the beds where I need the most compost - and move them about all the time.
Basically, dig, weed, plan and cover.