A raised bed is the quickest way to grow in poor ground. You build the soil instead of fixing it, and the bed warms up earlier in spring than the ground around it.
Start with the frame
Untreated larch or oak lasts a decade without chemicals. Keep the width under 120 centimetres so you can reach the middle from either side without stepping in, and make it at least 30 centimetres deep so roots have room before they meet the subsoil.
Fill it in layers
Branches and twigs go in first, then leaves and grass clippings, then half-finished compost, and topsoil on top. The woody base holds water and breaks down slowly, feeding the bed for years. Water each layer as you go so nothing stays dry in the middle.
Let the bed settle for two weeks before planting, and top it up with compost each autumn as the layers sink.