We had an allotment for three years before we sadly had to give it up when the nipper came along - not really any time to go down and put the work in.
A 1/4 plot will be a good place to start with. We had a 1/2 plot for the first year and then a full plot. If anything I wish we'd stuck with the smaller plot - easier to manage.
The best advice I can give, is that given to us by one of the old boys when we started. Plant lots of potatoes. More than you'll eat and give them away if necessary. The spuds really break up soil that hasn't been used for a long time with minimum effort.
After that, we roughly stuck to a three year crop rotation on dedicated beds. Google or the HDRA will help with that.
Finally, obvious though it may sound, grow what you want to eat. Fresh sweetcorn can't be adequately described by me in type! Likewise freshly dug spuds. You name it, we grew it but personal favourites were sweetcorn, raspberries, rhubarb, mixed salad leaves French and broad beans.
I don't suppose our allotment was unique in that we were by far the youngest tenants but everyone was always keen to offer advice and answer questions we had.
Ask away with any specifics.
" Walter was a worm of very few words."