Hi Shelley - chances are the plot has only been vacant a year ... the plot I gave up only last year hasn't been taken on and you'd think I'd never weeded it in all the years I had it. Grasses and nettles so tall, you'd got lost in it LOL.
Good thing about nettles is that it is a sign of fertile soil
Personally I would strim the whole lot down first (see if you can borrow a petrol strimmer as the stems are likely to be a bit woody by now) and rake everything into a big pile and let it naturally compost down and you might be able to start to using it next year (don't worry about weed seeds at this point - your plot will already be full of them anyway so just take it in your stride and not stress about it)
If you go down the weedkiller route, let the strimmed bit get a bit of growth on (I know - scary, but essential) as most weedkillers will only be effective on actively growing plants (most plants usually bounce back quite quickly to get new leaves out and strimming it first just makes it easier to put the weedkiller down and use less of it) Choose a day with very little wind as you don't want to upset your neighbours by accidentally contaminating his/her plot, and a day when there is unlikely to be rain for at least 6 hours but longer would be best so that the weedkiller is getting to work) It can take about 2-3 weeks before you see progress after using weedkiller, and where there are no signs of die-back, give them another treatment. Be aware that weeds will still come back, and probably with a vengeance as you will have opened up the area allowing more light through to the soil and this will trigger all those weed seeds on the surface to germinate. They are usually easier to deal with (new seedlings can be hoe-ed or strimmed) but the trick is to keep on top of them. Things will slow down rapidly after the first frosts about mid-October so will help you to dig and clear the ground without the weeds popping up quite so quickly.
If on the other hand you want to avoid weedkiller, then this will take longer. I would suggest you cover areas with something like black plastic to exclude light and this will slow down their growth until you are ready to clear the ground. Make sure you dig up the roots of perennial weeds like dandelion, dock, nettles and couch grass (as well as the notorious bindweed and marestail if you are unfortunate to have these on your plot)
Read your allotment site rules that should be with the tenants agreement - it should tell you how much of the plot needs to be cultivated within a certain period, and what structures you are allowed/not allowed to erect (some can be quite fussy) Get to know your neighbours - they'll be a source of information such as where to get manure and if there is a garden association (my site has a couple of sheds where seed, canes and fertilisers are so much cheaper and more convenient to buy, as well as the potato catalogue due in a short while because orders generally go out in October/November before the site emtpies over the winter months)
Most important of all, start an early Christmas wish list for all those tools you'll suddenly find an interest in - a good quality spade and garden fork, maybe a wheel barrow, hoe, rake ..........