Ideally first job to double dig ie two spade depths deep. One time only job

Start a compost heap and add all vegetation from the garden and kitchen. This can be used as a soil conditioner to be dug in this winter. Wont need to spend money on buying manure etc if you use your own garden compost.

Grow crops on rotation ie do not plant the same family on the same ground repeateded.

In winter, apply lime to the area where you will plant your brassica's

You will not achieve ideal soil conditions immediatley, it will take some years to develop strucute and fertility but doesnt mean to say that you wont get good results now.

During summer, spread lawn mowings down the rows to preserve moisture and then dig into soil during autumn digging.

Regards,
Old Perennial