Hello Leanne - how exciting for you… I got an allotment 4 years ago, and I’d never grown any veg before other than tomatoes in a grow bag in my back garden.
Here are some tips:
Start on just one part of it, rather than trying to grow things all over.
Talk to other plot holders - they’ll be happy to give you advice / seedlings / details of seed swaps etc.
Think about ‘no dig’ - check out Charles Dowding’s videos - even if you don’t do no dig he has loads of brilliant advice from sowing seeds to making compost and everything in between
Having good soil is the key - so this autumn put loads of manure on your plot - or part of it! That way things will grow really well next year
Buy a book on growing veg on allotments - loads around incl. second hand… they’re good for getting an idea of what to plant when, different varieties of veg, pests, etc. etc.
Do you like growing things from seed? Some people do (I love it) others don’t, and so buy seedlings… research where you can get seedlings if you don’t want to grow from seed - sometimes easier to start off
Don’t worry too much about rotating crops
Plant what you like to eat (might sound obvious, but you might be tempted to grow, say, beetroot, because everyone else is, even though you don’t like the stuff
After a bit you’ll work out both what you can grow, and what you like to grow e.g. I can’t grow carrots for some reason, and don’t bother with onions, but grow loads of chard, potatoes, rocket, kale, beans, leeks - amongst many other things
Things you can plant in the late summer / autumn include garlic, broad beans, lettuce, kale, chard… so it’s not to late to get stuff in the ground.