Onions, whether sets or seed require a sunny, well-drained site.. If you heavy soil the drainage should be improved prior to planting with grit and bulky organic matter, and the cloves planted in ridges of soil 10cm (4in) high to help reduce soil moisture.
Onions like fertile soil, but don't require much nitrogen and so shouldn't be grown on freshly manured soil. Instead, dig over and manure the ground several months before planting, i.e. now for sowing sets in early spring. If the soil is acid it's worth liming it so its pH level becomes neutral or even slightly alkaline.