Ideal pH: 6.0-6.8. The softer and more humus-based the soil, the better. When soil is dry enough in spring, work it to a fine texture. Broadcast and dig in ½ cup complete organic fertilizer for every 3m (10') of row. Avoid fresh manure. Carrots will become misshapen, but still edible if they hit anything hard as they grow down into the soil. Keep weeded and watered.
It is very important to thin carrots in order to allow them room to grow, and so they don't compete for available nutrients, moisture, and light. Then to 4-10cm (1½-4") when the young plants are 2cm (1") tall. Use wider spacing to get larger roots. As they grow, carrots push up, out of the soil, so hill soil up to prevent getting a green shoulder.
Here are some more good tips on how to grow carrots from seed!