Congratulations!
We've had two allotments over the years, our current one we've had for 5 years. To add to the others:
I agree grow what you like eating or using (cut flowers/woodies) and where possible grow the things that are expensive in the shops. For us that's mainly fruit and herbs bt we grow veg we like too.
Go up at different points in the day to begin with to see how the sun falls, whether it's colder or warmer etc. This will help you plant your crops, there are plenty of edibles that suit shade but tend to be grown in full on blazing sun for example. Go up after rain too to see if it is sitting on the soil or waterlogged in some areas, or indeed very dry.
Check your soil structure and type. Clay and loam types will be different to grow with than sandy. pH is often touted as very important but most plants are actually quite flexible on pH unless you are growing full on acid lovers like cranberries and blueberries. Allotment soil structures can be terrible and need remedial action depending on the previous tenants but as you have raised beds you have less to worry about as you can always change it.
Check for organic matter in the soil and worm activity. Our plot was very well tended or so it seemed, when I checked for organic matter I realised it hadn't had anything other than pour on fertilisers for years. Worms are a great indicator of good soils, I aim to mulch for worms are they not only compost everything they aerate the soil too which in turn allows for better water penetration.
Make sure you are appraised of the terms, for example do you have to keep it weed free, 80% cultivated etc? If you are ill or on hols for more than 5 days inform the person who manages the allotments, it can save you getting a letter to vacate when your perfectly weed free soil gets some rain and suddenly a whole load of annual seeds decide to sprout and grow like mad. This time of year two weeks can look like no one has visited for 6 months.
Make friends with the neighbours, some can be very helpful but sometimes helpfulness is not always helpful. Watch out for the guys who live up there, they will be very useful to tell you what grows well and what doesn't but they can also be a bit controlling as well.
If you practice slightly alternative method of growing like no dig, rather than smashing everything with pesticides and digging trenches for battallions on the Somme the allotment, be prepared for people to tell you why you are wrong without ever having engaged in a conversation about it. I just smile nicely and do my own thing. I'm a qualified agriculturalist and worked in agriculture and horticulture but as a woman in her 50s, I obviously don't know anything about it like the life long accountants and HGV drivers do.
Plot permenant paths so you can move easily between beds, it makes like a lot easier than having one massive bed and you can get your rotation planned and growing.
Plan your rotations at least 2-3 years in advance taking all the notes from your soil, climate, and aspect information above to grow the right plants in the right places and reducing the risk of disease.
Companion planting and catch crop sowing is great on smaller plots, it maximises the amount of plants for the space. Also plants love growing with other plants, some have longer root systems and draw up the minerals and nutrients other plants utilise. Some cover the soil with leaves to keep soil moisture locked in and available.
Sorry I only meant to write a few things but got excited there. One last thing about watering. One of the biggest mistakes I see in gardening is watering schedules or rather the lack of them. Once the plants are established they don't need huge amounts of water yet I see people watering every day, this allows for very poor growth, insipid tasting plants, and insect attacks because they love a juicy plant full of water. I water once a week and I know how many watering cans per crop per bed. That is unless it is going to be over 24C, then I pop up in the morning and give them a water but not a soaking. There are debates on morning or evening watering, as usual it's up to you how you want to do it.