I always start with potatoes - they are great for clearing up a new plot.
Clay is not the best - you might find they rot. but I have no choice, we are on clay, some is the proper orange stuff you can make pots from!!
Spinich grows well here, and most brassicas.
I wouldn't bother with any root veg (parsnips, carrots etc) until you've worked the soil.
Good luck, and at least you will find good use for all the animal manure
