Here is a quick introduction to crop rotation, I'm not promising your crops will be perfect year after year but these are principles that will definitely help out along the way. The main principle is to plant crops in a different position each season/year, ideally you should have a three- or four-year rotation.
For instance, I have grown broccoli and cauli in my small timber raised vegetable garden this year, but will plant alternative vegetable varieties/families in this particular garden for the next two years only returning brassicas to it during the winter of the fourth year.
The brassica family which includes cauli and broccoli strip a lot of nutrients from the soil, so I intend to select light feeders such as onions and peas for this garden in the future.
The reason for this is it helps to maintain or enhance soil fertility as some crops are heavy feeders, meaning they use a lot of nutriens in the soil during a single growing season. Some crops are light feeders and some help to rebuild the soil so changing the crop variety will balance the health of your garden.
If you have had a disease or particular group of pests in your garden effecting a particular plant variety, the rotation process will also help to eliminate the disease and reduce the pests before a large population of spores and eggs accumulate on the same plant variety again. You can group your crops into families: Brassicas/Leafy vegetables - Cabbage, cauliflower, Kale, radish (lettuce has fewer problems so plant anywhere) Roots - Beetroot, celery, parsley Legumes - Peas, broad beans (Runner beans suffer from fewer soil problems and can be grown anywhere)
Root crops - Onion, garlic, leek
Potato family: Potato, tomato (Capsicum suffer fewer problems and can be grown anywhere)
Work out a crop rotation you can use, this simple diagram is for a four-year crop rotation, returning year ones crop back in the fifth year.
As a general rule brassicas follow legumes. Legumes fix nitrogen in the soil so it is nutrient rich for the introduction of brocolli and cauliflower.