welcome to GoY :o)

if you have removed all the roots then you need to dig the plot over, removing any large stones as you go. Any roots left in will regrow brambles. Any large stems left in can be dealt with a herbicide like a brushwood killer. SBK is an example of this type.

What you grow will depend on what you like to eat.

if you want carrots/parsnips then the soil wont need much doing to it, but the other crops will need compost/rotted manure/ growmore granules adding to improve fertility of the soil.
the cabbage family prefer a neutral to alkaline soil so if it is acidic you will have to lime the soil.
peas/beans will be fine as long as there is rotted manure/compost added to the planting holes when you plant them.

Congratulations - sounds like a mammoth and unpleasant task. Don't be too despondent if some brambles do still appear - they are determined beasts. You might consider a cropping plan that will allow you to clear the ground at reasonable intervals through the first season.I'd be tempted to grow some first early potatoes. By the time they are ready any brambles left should be showing and you can remove them. Follow the potatoes with a winter leek crop which you can be growing in trays or cells ready to go in later.. Some quick growing salads, lettuce, rocket, spinach etc.
Then you could have a nursery patch for winter brassicas that can be planted out in a cleared patch later in the season. You haven't said where you live - if its reasonably mild in winter you might consider some broad beans for overwintering in a poly cloche tunnel - Aquadulce is a good reliable variety for that purpose.