congratulations ! Im in year 3 of my allotment and only really getting into ' full production'
Its a bit late to plant most things now for summer eating though it could be worth chucking in some peas and beans and see what happens.
I'd focus now on getting the plot cleared, if its overgrown, and finding out whats already on there. We inherited a lot of raspberry canes and redcurrent and blackcurrent bushes.
Then decide what you want to grow. It sounds daft to say it but grow what you want to eat. We like soft fruit so have a huge strawberry bed and added more raspberries. We put in a dwarf apple tree which will give us the first fruit this year. We also eat lots of peas so grow petis pois and mange tout. I have invested in a polytunnel for salad crops.
We are very excited this year as we found a grapevine in a very sorry state in the weeds. With a bit of TLC it produced one small bunch of grapes last year, this year looks more promising.
You might want to think about a wild flower area to attract bees and butterflies. We are setting up a habitat this year for frogs and newts.
Most allotment sites have one or two old geezers who practically live there and are happy to dispense wisdom and are worth getting to know.
We have very poor soil and as I havent got the time or patience to dig in mountains of horse poo to improve the quality so I've gone for raised beds. We have someone on site who knocks up raised beds out of pallets for £20
Think about it as a long term project. I have a book called the Half -Hour Allotment by Lia Leendertz which I've found helpful.
As far as time goes, I probably spend a couple of weekends at the beginning and end of each season to get things set up, plant seeds and cleared away, and then probably 1 to 2 hours a couple of times a week.
I'd like to spend more time, grow more stuff and learn how to bottle and preserve, but thats for the future.
Have fun!