If your plot is very overgrown don't despair! But be realistic, you need lots of time and energy to clear a very bad plot.  Consider taking on just half for the first year and let the council keep the rest, or take the whole plot but cover the bit you cannot work straight away (an old silage sheet is ideal). You can find more information on suppressing weeds here.  Most people think you only have to dig and weed once, but a very weedy plot will have to be dug many times to remove all the weeds and seeds you missed the first time, more seeds will germinate as you expose them to light.

If your plot is weedy in certain areas. Assess how much time you have and whether it is realistic to get it all cleared right away. Concentrate on the bits you can cultivate and cover those you can't, with well anchored black plastic.  It's a good idea to remove rubbish and to strim the area so the plastic stays flat

If your plot is well cultivated. You are very lucky! The plot will probably have been divided in to three or four areas, so try to work out what was previously grown in each area so you don't end up growing cabbages two years running in the same place.  Neighboring plot holders should be able to tell you how the plot was worked before.  Based on this information work out how you are going to grow things, it doesn't have to stay the same, it's your plot now!  If this is new to you find more information on crop rotation here.