Blimey, so few words and such a big question! It's all in the preparation - of the ground I mean, which should have been dug thoroughly, removing any rubble, debris and deep weed roots, then levelled, then walked over to get out soft spots, then relevelled, then raked over and feed applied, then the turf goes down. If you're not sure, beg borrow or steal yourself a copy of Dr. D. G. Hessayon's The Lawn Expert - tells you exactly what to do and gives more information about lawns than you're ever likely to need, and costs about a tenner, though often cheaper on ebay or similar, sometimes local libraries have copies.