Shame that the composter has never been used
I'm not going to urge you to 'double dig' because that would be cruel

but really it would be your best bet on a new plot.
You can probably get away with a 'single dig' tbh. Work in rows and dig a spade's depth along each row, breaking the soil up a bit as you do. At the end of each row, go through the piles you've just dug and get every last root and stone you can out of them and further break up any big clods. You'll see some roots going off from the trench you've just dug too no doubt. Try to get those out too. The more you can get out the better.
After that, put a good spadeful of whatever organic matter you can get into the trench. (Asking the locals what they use would be a good idea).
Then step back and start a new row. This time you turn the soil into the trench you just dug. Again taking out as much root as you can and breaking up any clods. Keep going back like that till you've dug over the lot, added matter to each trench and got every last bit of root you can out. The last trench is filled with the soil you dug out of the first one. This is the basic 'single dig'
You WILL get weeds come back, it's pretty much inevitable but effort put in at this stage will make life easier down the line.
What you do after this is up to you. I'd recommend a clearing crop and green manure crop like I've said earlier but you could just go straight for growing some food. Be aware though that root vegetables don't like growing in freshly mucked soil. They hate it in fact. The bean family, most salads, capsicums etc will do well in newly mucked ground though.