A sandy loam is ideal, as are raised beds. Start by removing the worst of the stones. Break up any stubborn clumps. Heavy clay is not great for raising carrots, but if that's all you have, don't fret. Frequent applications of manure or compost will quickly remedy the situation.
Carrots also like to follow a crop that has been composted or manured the previous year.
Avoid applying fresh manure or compost right before sowing, as it tends to make them tough and watery. Six seeds per inch is recommended but that's easier said than done. To make sowing more manageable, empty the seed packet onto a sheet of stiff paper. Pinch the paper along one side to form a creased wedge and tap out the seeds. You can also purchase pelletized seed and seed tapes.