Have a look at your plot and make a plan of where you would like to put stuff. best thing to do is go to the local library and borrow some veg books (alan titchmarsh) and some allotment books. They'll give you some layouts of allotments to look at and see what type of thing you would like/manage with.
You'll need to know which way north/south is etc to get the best light and also if it is on a slope etc. Have a look at what the other allotment people have done and see if you can go down and have a chat. 99% of allotment holders are friendly and will give advice.
You'll need to know whether anything has been grown on it before or if not how long has it sat un-used if possible.
If you want to plant this year, I think you'll need to mark your plot and cover it over or take the top layer of grass off. Some Rotavtors will just chop the grass roots up into the soil and make more weeds.
Potatoes are always good to go into the soil and they break it up nicely, you'll need to do potatoes soon though. They don't mind what type of soil really.
You'll need to know what PH your soil is and can buy a kit for this at your local garden centre. This will tell you what you can plant in the soil and if you want to plant anything else you'll need to add to the soil - sand, compost etc.
Yes a compost bin and water butt will be acceptable but decide where you want them first.
Good Luck it's good fun. I've got a plot full of clay soil and so we've only got potatoes, horseradish, garlic and brussels sprouts this year in the ground. I've put a few other things in pots for this year whilst we get the plot sorted out.