Sorry - a ton of questions (new allotment)

Spend a while down at the allotment today clearing weeds and shopping trolleys (!?!) and have measured it up. The whole allotment is 13'6 x 41 (but only 36' of that is useable, the last 5' is next to a weird fence with blackberries growing over and it's quite wonky and sloping). In the middle there is a 8' x 8' pen, it has mesh walls and I think a mesh cover that's folded back - first question  I guess is what would that be used for?
Secondly, I have done some scale drawings and looking at the allotment I can fit in 4 x ( 3'6" x 5'9") beds at the bottom (the whole space before the pen is 13'6 x 9ft, but I read that 4' x 4' beds are best to work with (?) so have added in some paths, dividing into quarters.  To one side of the pen is a 4' x 8' bed, and to the other side is 2 small 3' x 2' beds. Then above the pen there is room for either 2 or 4 4' x 5'9" beds (depending how much space is pinched by my mum for flowers). Then (if 4 beds are used) I have a further 9ft, but that is definatly flower space (unless veg. space is too tight). The problems I have is - are the beds too small? Too big? What could go where? I have tried learning the crop rotation stuff but I am not sure I totally understand it all, about keeping potatoes away from something else etc - how would I rotate round these beds? What do I need to do to each bed for each veg. ie. manure/no manure compost/no compost (I was just going to double dig them all and add organic compost). The veg I was thinking about having (total novice) are
asparagus (?)
cabbages (2-3 types?)
radishes
potatoes (2-3 types)
lettuce
cucumber
blueberries (?)
mustard (?)
watercress
avacado (?)
sweetcorn
peas
some sort of beans
celery
garlic
onions
spring onions
carrots
strawberries

quite a list, not sure if half of it will grow though. I would be so grateful if you could help, it's so complicated!
Thanks in advance x

Sounds like the 8x8 pen was some sort of animal enclosure, possibly chickens or young children
As for your first year, I'd suggest you simply compost the whole area, add manure if you can get some, mostly in the area for potatoes and just get started. Things will eventually make sense to you. It took the rest of us a while to figure things out so don't feel confused. All of what you list should do fine in some compost.
As for asparagus, they're perennial and will last many years so place them out of the way of your regular beds or rotovating or whatever you eventually plan to do. Add a bit of lime for them.
Oh...garlic should be planted in fall for summer cropping. You'll probably miss this year, but you'll be ready for next.

Hi there. I had just about finished typing a reply when I lost my connection, so here goes again.

The enclosure might be a fruit cage or it might be what we did when we first got ours. We cleared a small space just to grow something quickly before clearing the rest of te plot. It could be rabbit defences if the mesh is buried about 6 inches and about 3 feet high.

You will find lots of advice on here but the main thing is DON'T PANIC!  

I know it's easy to find it all a bitdaunting at first.

Just a quick thought ... Blueberries are VERY partial to acid soil ... I grow mine in containers so I can look after them better  

And if you are short of space grow french (bush) beans interplanted with your brassicas they like each other.    
And amongst your sweetcorn ... that you should plant in a block underplant with rambling squash and even grow climbing beans up the corn !!  These are often called "The Three Sisters" !!  Companion planting at it's best  

Asparagus is a good one because it's dear to buy in the shops. Buy some right now and get it in quick, it's the right time. Weed your bed well. Wait 2 - 3 years for first crop then you'll get it every year.
Fruit bushes and trees can be put in now too. Space them well, even though it will look a bit sparse right now.
For bed sizes - if you want to cover the soil with stout woven weed suppressant at any time, best to make the beds the same width or slightly narrower than the standard sizes that weed suppressant comes in. Like 1 metre or 2 metres wide.
Onion sets can be planted now, and garlic. For the first year I wouldn't bother with cabbages or celery or mustard, because they are cheap enough in the shops and you've got enough work on at the moment. As has been said, avocados won't grow, and blueberries need ericaceous compost or naturally slightly acid soil.
Don't get too wound up on rotation - just learn whats in the rotation groups and avoid growing those in the same group in the same place year after year.
Sounds like you've got a fruit cage if it's high enough to walk into. If the side wall mesh is big enough for insects thats fine, if not it's useless. If the basic structure is good, think twice before pulling it down, maybe you can re-cover it if the mesh is too fine.
Best of luck.

wow this is great, I'm slowly reading through all the posts here and found this one very informative.
my plot will be new, so nothing on it at all, will need to start collecting stuff together, esp a prized chair to watch my lovlies grow - lol x