Remove all stones and rubbish from the lawn area, you can now start to prepare the ground. The better prepared seedbed, the better the lawn will be.
Good drainage is important and deep digging will help. If the soil is heavy add more sand this will improve the drainage. If the soil is light or sandy, work in a good amount of peat or compost to give body to the soil and help prevent drying out and loss of nutrients. Any levelling should be done now, take care not to remove too much topsoil from any one area or the results could be areas that vary in growth and colour.
Ideally the initial digging should be done in the Autumn so the soil can be left "as dug" in large lumps as possible, over the Winter rain and frost will break down the soil making it crumbly.
In Spring when the soil is drying out, lightly roll or tread the area down and rake, first one way across the area then rake and firm again, rake and firm until a firm and even seedbed is acheived. Apply and rake in a gereral pre-seeding fertiliser, this will stimulate root growth and provide the essentail early feed to get the lawn off to a good start.