Ask the other plot holders and get lots of good and bad advice. Buy a book that works through the seasons was pretty good for me. Got an overgrown mess of an allotment last year and am now enjoying bumper crop of spuds, onions, spring onions, corgettes, rhubarb, green beans and soon sweetcorn, oh and lots of salad. Im using seaweed off the beach as fertiliser and our allotment did get a grant for a load of compost. The local B and Q is also more than happy for us to raid there garden chuckaway for cains and pots etc.
Get a spade and dig, learn how to backfill!