It is vital to have a vision of what you want to achieve with your new garden. Your vision includes what sort of plants you want to grow. A friend of mine specializes in flowers. It includes whether you want to be a competition grower or not. Some ethnic minority growers grow favoured foods in their own ancestral lands. I have heard of a woman who grows herbs for medication and cosmetics. A vision should also include how you want the garden to progress, for example in two years time.
But your task begins with clearing any rubbish left by previous tenants of the garden. My son has to move an amount of scrap wood left by his predecessor. He is depositing it in a dumpster (skip) hired by the allotment management. But this is prior to clearing any weeds. Some weeds are baddies. Ragwort is a nuisance, as is creeping buttercup, which flourishes in the damp soils of north west England, and mare's tale, a Sicilian invasive, takes a long time to clear. These, along with others, need clearing. Weaker weeds can be tackled when hoeing the surface.
Get a good knowledge of your soil. This is vital. Find out whether the soil is clay, sando or silt, or as is most common, a loam, which is a mixture of all three. It is a good idea to find out the pH of your soil, whether it is acid, alkaline or, more rarely neutral. Acid soils are pH I to 6.5. Alkaline are 7.5 to fifteen. A pH of 6.5 to 7 does not need remediation. Knowing the pH helps if you want to grow acid loving plants. Some vegetables,such as cabbage, prefer a slight degree of alkalinity, a pH of 7,but they can tolerate some variation.
You also need to check the amount of organic matter in the soil. Ideally your soil should be dark,th le sign of a soil with a good level of organic matter. Organic matter is provided by compost,well rotted manure or seaweed. Leaf mold can be added, but it is not high in nutrients. Don't add fertilizer until you are ready to plant, when some can be added as a base dressing. Sometimes growers sweeten their soil with lime, but doing this is not always necessary, and as pH rating is on a logarithmic scale, moving from pH 6 to 7 is ten times harder than moving from 5 to 6.
It is useful to ask other plot holders whether there are any plant diseases endemic in the area. Club root is a common fungal disease of brassicas (cabbage family) and the only defence is a good dose of lime,but that is a preventative not a cure. Onion white rot is a bad guy. There is no cure, except to not grow onions on the infected bit for a long time. Silver leaf is a common disease of the plum family, I had to fell a much loved damson to get rid of it. These are just a few examples, so a good book on plant diseases would be useful.