Brassicas include: broccoli, cauliflower and cabbage
Dig the soil over and add organic material such as garden compost in the winter. When early spring arrives dig the patch again and sieve the soil so that it has a fine tithe to it. Add lime to the surface of the soil and fork it in. However, there is a remote possibility that your soil is already alkaline - in that case adding lime isn't necessary.
Start growing in a greenhouse or on a window ledge
Growing seeds in seed trays and putting those trays in sunny warm conditions helps them develop quickly, so I highly recommend using a greenhouse or putting seed trays on the window ledge, on the sunny side, of a house. Once the seeds have developed nicely they should be transplanted into the patch.
Regular watering is required during dry spells. Use a watering can, but if you have a hose pipe using that is far easier and convenient.