if your plot is thickly grassed it could work in your favour, to a certain extent at least. You may want to consider hiring a turf cutter, lay out your planned beds, strip them off, stack the sods face down to rot, cover the beds you can't use striaght away, and start digging those you will use, removing the weeds as you go. Mow off the areas left as paths and your on your way. Your plot will be laid out, your paths in place plus you'll have a good compost heap started. If you have perennial weeds on the plot, cover the compost heap with black plastic and leave it for a good twelve months, better still two years to make sure all the perennial roots are killed.
The remark about not attempting too much is very good, perhaps the best piece of advice you can get. I took on my plot last may, which was meadow with a healthy crop of bramble, nettle, dock, bindweed and couch grass. Sadly the site was ploughed before we took over. All that did was to turn all those nasties into the soil and make clearing it much, much harder.
Two of the people who took plots at the same time as me tried to clear the whole lot straight away, but decided it was too much for them and gave up.
On the other hand my plot looked like I had given up. I visited once a week, maybe only once a fortnight and did a bit of strimming, and weedkiller once a month, planted some runner beans and other bits and pieces which grew through the resurgent weeds.
I still managed to get a good crop of runner beans, a few peas and the odd cabbage, but decided to forgo winter crops in order to cover the whole plot. That is paying dividends now, though it will be a few years before I have the allotment how I want it, I am at least ready to grow everything I want to in my first full year.
OK, a fair chunk is being given over to potatoes, plus my brassicas, peas and beans are all being sown into peat pots and will planted on through weed membrane. Whats not being used will stay covered, gradually this policy will hopefully starve the weeds, though I doubt I will ever beat them completely.
What I am finding is each time I go down and do a bit, it gets easier. As an office worker not used to much hard work it was a real effort to begin with, but as I get more used to it, so it gets easier - and therefore more enjoyable.