Your snail tank should have at least two inches of damp compost or moss for your snails to burrow into. Popular substrates are compost, coconut coir (coconut fibre) and spaghnum moss. These can be bought at pet shops that cater for reptiles, garden centres or online on sites like eBay or Amazon. You should make sure that compost has no chemicals or additives in which could harm your snails.
Substrate should be cleaned or every month or so to keep the tank fresh. Some substrate can be re-used by baking it to kill any bacteria - Coir can be baked at 200°c for one or two hours, left to cool and then dampened before returning it to the tank. Sphagnum moss can be cleaned by rinsing it through with warm, not boiling, water.
As well as substrate, you can make the tank more interesting for your snails by adding pieces of driftwood or cork bark for them to climb on and hide under. You could also try adding artificial plants (plastic, not silk) for more decoration. Avoid hard objects, such as terracotta flowerpots or rocks, as snails can fall from the sides or top of the tank and crack their shells on them.