Sphagnum moss mixed with peat moss. It is also a good idea to mix in a little bit of silica sand or perlite to make the soil a little more airy. You can also mix in a little charcoal to remove salts from the soil. Use the tray watering method. A very good beginner plant.
Most sundews are easy to grow if watered on the tray system.
This will provide enough local humidity for most sundews which form flat rosettes, like the one in the picture on the right. Some of the species that grow upright need more humidity than what is normally in the air of your house. Some sundews require a Winter dormancy period. It varies with most species so a book would be a good thing to get. Basically though they will die back and form a rosette of small non-sticky leaves.